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ASE Brakes > Air Brake Diagnosis and Repair > Flashcards

Air Brake Diagnosis and Repair Flashcards

1
Q

When will some models of antilock brake systems (ABS) turn off the dash warning light?

A

Will not turn off the dash warning light until the truck exceeds approximately 6 mph.

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2
Q

When do other models turn off the ABS warning light?

A

After a brief self-check and do not wait until the wheel speed sensor signal is received.

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3
Q

What specific federal legislation defines system air buildup?

A

FMVSS 121. This legislation defines the required buildup times and values.

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4
Q

What does a common check performed by enforcement agencies require?

A

The supply circuit on a vehicle is capable of raising air system pressure from 85 psi to 100 psi (586.1 to 689.5 k Pa) in 25 seconds or less.
Failure to achieve this buildup time indicates a worn compressor, defective compressor unloader assembly, supply circuit leakage, a defective governor, or restricted intake.

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5
Q

How would you test the air system buildup time?

A

The brake pedal can be pulsed until the pressure is below 85 psi, then time the buildup from 85 psi to 100 psi.

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6
Q

How would you check air system loss rate?

A

Build the system pressure to governor cutout, shutting the engine off, and timing the pressure loss.

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7
Q

Describe how you would perform Test One.

A

Allow the pressure to stabilize. Watch the dash gauge pressures for 2 minutes and note any pressure drop.
A straight truck or a tractor without a trailer is allowed a maximum pressure drop of 4 psi in 2 minutes in either service reservoir.
A tractor-trailer is allowed a 6 psi pressure drop in 2 minutes in either service reservoir.
A tractor with 2 trailers is allowed an 8 psi pressure drop in 2 minutes in either service reservoir.
If the pressure drop limits are exceeded, the technician must locate the source of the leak. If test results are within specifications, the technician can move on to the next test.

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8
Q

How would you perform Test Two?

A

With the air system fully charged, the engine shut off, and the parking brakes fully released, the technician should make and hold a brake application for 2 minutes.
A straight truck or tractor without a trailer cannot exceed a 6 psi pressure drop within 2 minutes in either service reservoir.
A tractor-trailer cannot exceed a 9 psi pressure drop in 2 minutes in either service reservoir.
A tractor with two trailers cannot exceed a 10 psi pressure drop in 2 minutes in either service reservoir.
If the limits are exceeded, technician must find the source of the leak.

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9
Q

What must both the primary and secondary circuit air pressure be monitored by?

A

Both the primary and secondary circuit air pressure must be monitored by a dash-located gauge; this can be two separate gauges that has two hands (one for each reservoir.

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10
Q

How can you verify air pressure gauge operation?

A

Air pressure gauge operation can be verified by using a master gauge, a good-quality, liquid-filled gauge that uses a Bourgon principle of operation.

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11
Q

Should you rely on the vehicle gauges, when troubleshooting vehicle air pressure management problems?

A

The vehicle gauges should not be relied on when troubleshooting vehicle air pressure management problems.
A failed in-cab pressure gauge may read too high, too low or become sticky.

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12
Q

Where do air pressure lines hab a tendency to leak?

A

At the quick-connect fitting. Often the technician can remove the line from the fitting, shorten the line about 1/4 inch, and reinstall. Re-ending the line in this way will often cure the leak.

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13
Q

How do pressure sensors (transducers) use air pressure?

A

To move electrical contacts along a variable resistor. This varied voltage will produce the signal for the pressure readings of electric gauges or warning. devices.

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14
Q

How do you test pressure sensors (transducers)?

A

A technician can disconnect the sensor and measure resistance across the sensor terminals. If the air pressure at the sensor changes there should be a corresponding change across the terminals.

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15
Q

What does FMVSS 121 require?

A

That a driver receive an alert when the system pressure drops below 60 psi.
In most cases, this is a visual alert accompanied by an audible alert, usually a buzzer.
A low-pressure warning device is fitted to both the primary and secondary circuits.
This is a simple electrical switch that can be plumbed anywhere into a system that requires monitoring.
The switch is electrically closed whenever the air pressure being monitored is below 60 psi.
Then the air pressure value exceeds 70 psi to 75 psi, the switch opens.

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16
Q

What provides advance warning to the operator that the vehicle is approaching a low air pressure condition or that pressure is building to a safe operating pressure?

A

The contacts will vary between being fully closed and fully open in the range of approximately 60 psi to 75 psi.

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17
Q

How can the technician verify the operation of a low-pressure warning switch?

A

By pumping the service application valve until the system pressure drops to the trigger value.

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18
Q

What provides air for the truck air braking system?

A

Air reservoirs store and provide air for the truck air braking system.

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19
Q

What does servicing of the air reservoirs involve?

A

Servicing of the reservoirs consists of inspection draining the tanks and performing leakage tests.

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20
Q

How many air reservoirs do air brake systems typically have?

A

Air brake systems typically have three air reservoirs.

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21
Q

Where is compressed air from the compressor delivered?

A

To the supply reservoir. The supply reservoir is also known as a wet tank.

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22
Q

How does the supply reservoir feed air to primary and secondary reservoirs?

A

Through one-way check valves. The primary and secondary reservoirs supply the brake circuit with air.

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23
Q

How did the wet tank or supply tank get its name?

A

Because of the moisture that forms in such a tank when hot compressed, moisture-laden air cools and condenses on the inside walls of the tank.

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24
Q

Reservoirsmaybe equipped with either automatic or manual drain valves. What is recommended to keep the system free of contaminants and moisture?

A

Daily draining of manual drain valves is recommended to keep the system free of contaminants and moisture.
Automatic reservoir drain valves should be checked for proper operation on a daily basis.

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25
Q

What are the supply reservoirs equipped with and what it’s function?

A

Supply reservoirs are equipped with a safety valve, also known as a popo-off valve. This valve is designed to open at 150 psi. This protects the system in the event of a governor failure.

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26
Q

Air reservoirs on trucks are what kind of vessels?

A

Pressure vessels that are hydrostatically tested after manufacture. The inside wall is treated with a corrosion-protection coating.
Truck air reservoirs should never be repair-welded because they are pressure vessels and the internal coating may be destroyed.

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27
Q

What is a safe and easy leak detection for air reservoirs?

A

Soapy water can be used as a safe and easy leak detection method for air reservoirs. Bubbles should appear at the source of the air leak. Leaking tanks should be replaced.

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28
Q

What does the evidence of oil in a wet tank often indicate?

A

Indicates the compressor is pumping oil through the system.

Oil can damage valves throughout the system so the source of oil contamination must b determined and repaired.

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29
Q

What is excessive water an indication of?

A

Indication of failure to drain the tanks regularly, or that the air dryer is not functioning correctly.

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30
Q

Describe the function, operation, and inspection of some compressors found on smaller engines.

A

Some compressors found on smaller engines have air compressors driven by the engine using a belt and pulley.
Belt sets, pulleys, tensioners, and idlers should be routinely inspected for indications of wear and axial run out.
Belt tension should be set to specification using a belt tension gauge.
Cracks and nicks in a drive belt require that it be replaced.

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31
Q

What should you do when replacing the serpentine drive belt?

A

You should move the tensioner through its complete range of movement. The compressor pulley should be replaced if worn or damaged.

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32
Q

How should you inspect and replace compressor drive gear and coupling?

A

Most compressors are two-cylinder pumps that are balanced units; The typically do not have to be timed to the engined on installation.
The drive gear and coupling should be inspected for wear.
Worn teeth can cause a knocking noise and can indicate a misaligned compressor drive.
When replacing the compressor drive gear and coupling, ensure that the oil feed passage is correctly aligned, free from debris, such as excessive gasket sealant, and the gear teeth are not damaged.

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33
Q

What steps should you take for single-cylinder types?

A

These compressors are not self-balanced units and must be timed to the engine on installation. Ensure that the manufacturer’s service literature is consulted when installing compressors that must be timed to the engine.

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34
Q

Why do the oil inlet and return lines need to be thoroughly inspected when you replace and air compressor?

A

A restricted oil supply line can cause oil starvation to the compressor.
A restricted oil drain line can cause the compressor crankcase to flood and result in the compressor passing oil.

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35
Q

Why does the coolant lines need to be thoroughly inspected when you replace and air compressor?

A

Restricted coolant lines can cause the compressor to run hot and pass oil or be slow to build pressure.
Coolant lines can be restricted, kinked or start to deteriorate.
A damaged coolant line or hose should be replaced.
O-rings on straight thread 0-ring type fittings should be replaced.

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36
Q

What can a loose or missing compressor mounting result in?

A

It can result in the failure of the accessory drive cover.

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37
Q

What are the many different air valves used for?

A

They are to control, regulate, or modulate the air in a braking system.
These valves guide the direction of flow and control the amount of pressure in the air system.

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38
Q

What does the governor do?

A

Manages system pressure.
It monitors the pressure in the supply tank by means of a line connected directly to it.
This pressure acts on a diaphragm and spring within the governor.
The spring tension is adjustable.
The governor manages compressor-loaded and unloaded cycles.
This loaded cycle is the compressor-effective cycle - that is the compressor is pumping air.
The unloaded cycle is when the compressor is being driven by the engine but not actually compressing air.
It induces a charge of air through the inlet ports on piston downstroke and forces it out through the same inlet port on the upstroke.

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39
Q

What must happen to the compressor to put it into it unloaded cycle?

A

It must receive an air signal from the governor. This signal acts on the unloader assembly in the compressor cylinder head.

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40
Q

When a compressor is driven by the engine, what cycle is it in?

A

It is in its unloaded cycle.

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41
Q

What is the function of the unloader assembly?

A

The function of the unloader assembly is to hold the inlet valves open - that is off their seats.

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42
Q

What happens if the unloader signal is not delivered to the compressor unloader assembly?

A

High system pressures will result.

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43
Q

What happens if the safety pop-off valve on the supply tank trips (at 150 psi)?

A

This is usually an indication of governor or compressor unloader malfunction.

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44
Q

What controls the compressor-loaded and unloaded cycles?

A

The governor. It defines the system pressure.

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45
Q

What values are governed pressures in most trucks set at?

A

In most trucks, it is t at value 110 and 130 psi, with 120 psi being typical.

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46
Q

What is cut-out pressure?

A

Governed pressure is known as cut-out pressure, the pressure at which the governor outputs the unloader signal to the compressor.

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47
Q

What is cut-in pressure?

A

Cut-in pressure is no more than 25 psi lower than the cut-out pressure value.

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48
Q

What is the difference between cut-in pressure and cut-out pressure?

A

Ranges between 20 and 25 psi.

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49
Q

How do you easily check governor operation?

A

One method is to drop the air pressure in the supply tank to below 60 psi, and with the vehicle’s engine running, build the pressure.
A master gauge should be used to record the cut-out pressure value.
If it is not at the specification value and the governor is adjustable, remove the dust boot at the top of the governor, release the locknut, and turn the adjusting screw either clockwise or counterclockwise to lower or raise the cut-out pressure.
Then drain the tanks again and allow the compressor to build the pressure to the new setting.
If the governor is not adjustable, it should be replaced.

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50
Q

What is the adjustment to an air governor?

A

The cut-out pressure value.

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51
Q

What would you use to test for leaks in lines, hoses, and fittings?

A

Soapy water.

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52
Q

What happens if brake hose is not replaced to its original length?

A

Brake performance will be compromised.

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53
Q

What affects both the application and release timing of brakes?

A

The sizing of lines on both the tractor and the trailer.

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54
Q

What will correct air brake fittings have stamped on them?

A

DOT

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55
Q

What should the brake hose be securely clamped away from when installed?

A

From moving components

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56
Q

When reusing dry seal fittings, what should be inspected?

A

The nipples and seats should be inspected.

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57
Q

What happens when a brake hose fails internally?

A

Sometimes forming a rubber flap that can act as a check valve in the line, permitting air to flow toward a valve but trapping it there.

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58
Q

Where is a system safety valve usually located?

A

on the supply tank or on the air dryer. It is designed to trip at a pressure value of 150 psi.
This valve can be adjustable or nonadjustable and consist of a ball seat and spring.

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59
Q

What is the function of a system safety valve?

A

Its function is to relieve system air if the pressure builds to a dangerously high level, such as would occur when a governor failed.

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60
Q

How can system safety valves be tested?

A

By pulling the exposed stem and listening for air escape.

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61
Q

What feeds the primary and secondary reservoirs of the brake system?

A

The supply tank.

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62
Q

How can a one-way check valve operation be verified?

A

By draining the supply tank and checking back leakage.

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63
Q

Automatic drain valves can become plugged with sludge-oil and water residues and may require periodic cleaning. Should oil and excessive water be evident at the drain plugs, what should you do?

A

Check the air dryer and/or compressor.

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64
Q

Some automatic drain valves will have heaters. How can these heaters be tested?

A

The heaters can be tested with an ohmmeter. The wiring should be mounted securely.

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65
Q

How is moisture in the air system very damaging and how is it removed?

A

The airborne moisture condenses in the reservoirs a th compressed air cools.
Most current systems use air driers to remove moisture from the compressed air before it gets to the supply tank.

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66
Q

What is the first principle used to remove moisture from the compressed air?

A

The first type is the desiccant type. During the charge cycle, hot compressed air passes through a desiccant pack that absorbs the moisture and the dry air exits to the discharge port and the supply reservoir.

A governor cut-out, an air signal from the governor begins the purge cycle of the air drier.
This acts on the purge piston which exhausts any water collected in the purge orifice to the atmosphere.
The purge piston can be repaired with a reseal kit or can be replaced as a unit on most driers.

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67
Q

What is the second principle used to remove moisture from the compressed air?

A

The second type of air drier is the heat exchanger type. The heat exchanger type of air drier cools the compressed air to the point that the moisture is condensed.
Once condensed, it can be separated and dumped by means of a purge valve.

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68
Q

What should the technician ensure when replacing an air drier cartridge?

A

The technician should ensure that the all air system pressure is drained.

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69
Q

When air driers use heaters to prevent icing, what temperature should the thermostat control the heater to maintain?

A

The thermostat controls the heater cycles to maintain a temperature exceeding 45 degrees F (7.22 degrees C)

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70
Q

What happens when an air compressor fails and pumps it lubricating oil through the system?

A

The desiccant pack becomes contaminated and requires replacement.

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71
Q

How can you test the air drier for leaks?

A

Spray the suspect areas with a soapy water solution and looking for bubbles.

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72
Q

What kind of service application valve is in a truck brake system?

A

A floor or firewall-mounted foot valve, sometimes known as a treadle valve.

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73
Q

Describe the treadle valve and what it does.

A

The treadle valve is actually two valves in one.
The upper portion of the valve is the primary section and the lower portion is the secondary or relay section.
Each section has a dedicated feed and its own exhaust port.
The upper or primary section of the treadle valve is supplied directly from the primary reservoir.
The lower or relay section of the valve is supplied by the secondary reservoir.

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74
Q

How is the treadle valve actuated?

A

The treadle valve is actuated mechanically, by foot pressure from the driver. When the driver’s foot acts on the treadle valve, the primary piston is downward.
This movement first valve the primary exhaust port and then modulates air proportional to piston travel to:
1, Actuate whatever brakes are plumbed into the primary circuit. This is usually, but not always, the drive axles on a typical tandem drive tractor unit.
2. Actually the relay or secondary piston, located below the primary section to a dual circuit application valve
3. Act against the mechanical pressure (foot pressure) to provide brake feel.

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75
Q

How is the secondary or relay section of the dual-circuit application valve actuated?

A

It is actuated pneumatically by primary circuit air. This section operates similarly to a relay valve in that a signal pressure value (the air from the primary section is used to display a relay piston that then modulates secondary circuit air to whatever brakes/valves are located in the secondary circuit. In a typical tractor air brake system, this would normally be the front axle brakes.
The secondary section is designed to modulate an air pressure value to the secondary circuit, identical to the signal pressure.

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76
Q

What happens in an emergency application of the treadle valve?

A

Primary and secondary inlet valves are held open and full reservoir pressure is applied to each of the two circuits.

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77
Q

What happens in the event of a total primary circuit failure?

A

The dash low-pressure warning alerts would trip, and when the treadle valve is depressed, foot pressure would drive the primary piston downward until it mechanically contacts the relay piston to actuate the secondary circuit.

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78
Q

What happens in the event of a total secondary circuit failure?

A

The primary section of the treadle valve would function normally, however, the dash air pressure alert would trip and the vehicle would have to be brought to a halt using primary circuit source air only.

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79
Q

How would you test the valve for proper air delivery?

A

By using pressure gauges plumbed to the supply and delivery ports.

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80
Q

What does a leak at the exhaust port of the value indicate?

A

That the valve is faulty or that there is a leak somewhere else in the system and the air is bak-feeding to the normally open exhaust port.

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81
Q

How would you check the valve and fittings for air leaks?

A

By using a soapy water solution.

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82
Q

What results if the pedal does not return to the release position fully?

A

A sticking pedal can allow a small amount of air pressure to remain in the brake chamber and result in dragging brakes.

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83
Q

How many brake light switches do trucks with air brakes have?

A

Two light switches, either of which can illuminate the vehicle brake light circuit

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84
Q

Describe how the service stoplight switch works.

A

The service stoplight switch is a normally open, air-actuated switch plumbed into the service brake circuit. The switch is closed by a small application pressure acting on it.

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85
Q

How does the parking brake light switch work?

A

This switch is designed to close the electrical circuit when no air is acting on it and to open when air is charged to the hold-off chambers.

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86
Q

How would you inspect the harness and connectors?

A

The harness and connectors need to be inspected for chafing and lumps in the insulation, which would indicate corrosion in the wiring.
Normal wiring repair techniques would be used on the wiring.
The switches are typically replaced not repaired. The correct new part should be used.

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87
Q

Where does the trailer service air line originate?

A

At the tractor protection valve.

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88
Q

Where is a gladhand connector located?

A

A gladhand connector is located between the servc line on the tractor and the trailer service line.

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89
Q

How is air pressure supplied to the trailer service air line?

A

Though the brake application valve or the trailer control valve during a brake application.

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90
Q

How many double-check valves are in the air lines between the brake application valve and the trailer control valve?

A

Two.

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91
Q

What is one double-check valve combined with?

A

One of the double-check valves is combined with the stoplight switch and mounted on the tractor protecion valve.

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92
Q

What happens if air pressure is applied to both supply ports on the double-check valves?

A

The check valve will allow airflow from the highest pressure source.

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93
Q

What happens if air pressure is applied to both supply ports on the double-check valves?

A

The check valve will allow airflow from the highest pressure source.

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94
Q

What happens if the trailer control valve is applied by itself?

A

The double-check valves prevent air pressure from being supplied to the front and rear axle brakes on the tractor.

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95
Q

Which valve is supplied for only the trailer brakes to operate.

A

When the trailer control valve is applied, only the trailer brakes should operate.

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96
Q

If there is a loss of primary or secondary air supply on the tractor, what should still be operational?

A

If there is a loss of primary or secondary air supply on the tractor, the service brakes are still operational on the trailer.

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97
Q

What should the driver operate to apply the the trailer brakes?

A

The trailer control valve is operated by the driver to apply the trailer brakes.

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98
Q

What happens when the driver operates the trailer control valve?

A

Air pressure is supplied from this valve to the control port on the trailer relay valve.
The trailer relay valve then opens and supplies trailer reservoir air pressure to the trailer brake chambers.

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99
Q

What should you use to test the trailer control valve?

A

Trailer control valves can be tested with an accurate pressure guage.

100
Q

Can you rebuild a faulty valve and with what?

A

Faulty valves can be rebuilt with a service kit when available. Often they will simply be replace with a complete new valve.

101
Q

What supplies the trailer with air and how?

A

The trailer is supplied with air by means of air hoses.
Service and trailer supply air is plumbed to a tractor protection valve.
The air is transferred to the trailer by couplers know as gladhands.
Gladhands enable easy coupling between the tractor air supply and controls and the trailer pneumatic circuit.

102
Q

From what are gladhand seals usually manufacture?

A

Gladhand seals are usually manufactured from rubber. They are retained in an annular groove in the gladhand and are easily replace when they fall.

103
Q

What controls the relay valve and what is its function?

A

Relay valves permit a remote air signal from the treadle or trailer valve, to effect service braking with an air supply close to the brake chambers.
The valve is controlled by a signal that is plumbed to its service port.
At the supply port, system air pressure is available to the valve from a nearby air reservoir.
Then the relay valve receives an air signal, this pressure acts on the relay piston.

104
Q

What happens when the relay piston is displaced?

A

It modulates the air available at its supply port to actuate the service brake chambers connected to its delivery ports.

105
Q

How many brake chambers can one relay valve typically manage?

A

One relay valve can typically manage two to four service brake chambers.

106
Q

How is a relay valve designed?

A

The valve is usually designed so that the signal value is equal to the output value modulated to the delivery ports.
When the signal pressure is dropped or relieved, the relay valve exhausts the service supply air returning from the service chambers, and a retraction spring returns the relay valve to its neutral position.

107
Q

What happens when you have a sticking relay piston in the relay valve?

A

A sticking relay piston in the relay valve will cause supply air to be modulated to the service chambers when there is no signal pressure.
This causes dragging brakes or locked brakes depending on the location of the relay piston.
Moisture in the air system may freeze and prevent the relay retraction spring from returning the relay piston.

108
Q

deescribe the typical quick-release valve.

A

The typical quick release valve has a single inlet port and two outlet ports.

109
Q

What happens when air is charged to the inlet port?

A

The valve acts like a T fitting and simply and simply divides the air to the two outlets.

110
Q

What happens when the air pressure supplied to the inlet is exhausted at the application or control valve?

A

The air being outputted from the quick release to the brake chambers is exhausted at the quick-release valve.

111
Q

How may a quick-release valve be used?

A

Quick-release valves can be manufactured to incorporate a hold-off or crack pressure: care should be taken to replace a faulty valve with the correct replacement parts.
A quick release valve may also be used on the service gladhand supplying the trailer(s). This feature greatly reduces the release lag that can occur in service braking, by exhausting the service signal at a much higher speed.

112
Q

How can quick-release valves be damaged?

A

They are vulnerable to external damage because of their exposed location and contaminated air, which may cause exhaust port leakage.

113
Q

What prevents both the service and parking brakes from actuating simultaneously?

A

An anti-compounding circuit prevents the possiility of both the service and parking brakes from actuating simultaneously.

114
Q

How can you check the lines and one-way check valves?

A

The lines and one-way check valves can be checked by applying the parking brake and then applying the service brake and then applying the serivice brake.

115
Q

In a proper operating system, what will the air that is applying the service brake do?

A

Release the parking brake in a proportional amount. These valves are not typically repaired, They are replaced when they fail.

116
Q

What does the tractor protection valve do under breakaway condition or severe air leakage?

A

Protects the tractor air supply.

117
Q

what connects a tractor brake system with a trailer air circuit?

A

Two air lines connect a tractor brake system with a trailer air circuit.

118
Q

What does the trailer supply line supply?

A

The trailer supply line supplies the trailer with air for braking and for any other pneumatic systems such as its suspension.

119
Q

The trailer service line is the ________.

What are both of these lines plumbed to?

A

The service brake signal line.

The tractor protection valve.

120
Q

When the trailer supply dash valve (emergency) is in its off position, what position is the tractor protection valve?

A

When the trailer supply dash valve (emergency) is in its off position, the tractor protection valve is in its off position.

121
Q

What happens if both the trailer supply dash valve (emergency) and the tractor protection valve are both in its off position?

A

when the driver makes a service brake application, no air will exit the tractor protection valve service signal line to the trailer.

122
Q

What happens when the trailer air supply valve is open?

A

This air will open the tractor protection valve and supply the trailer with air.
In this condition, when the driver makes a service brake application application, the service brake signal air will be transmitted through the tractor protection valve to apply its service brakes.

123
Q

What controls the trailer park brakes?

A

The trailer air supply controls the trailer park brakes.

124
Q

Whenever the air supply is interrupted, whether intentionally, such as when the trailer is being parked, or unintentionally, such as in a breakaway, what will happen?

A

The spring brakes will apply regardless of how much pressure is in the trailer air reservoirs.

125
Q

How is the tractor protection valve designed?

A

The tractor protection valve is designed to isolate the tractor air system from that of the trailer at a predetermined value that ranges from 20 to 45 psi, depending on the system.
At these pressures, the spring brakes on bothe the tractor and trailer would be partially applied.

126
Q

How can you test the tractor protection valve?

A

The valve can be tested by draining the tractor air supply and watching for the button on the dash to pop out at the appropriate psi.

127
Q

What does the spring brake valve do?

A

The spring brake valve supplies air to the hold-off chambers to release the spring brakes and enable the vehicle to move.
During normal operation, it limits the hold-off pressure to the spring brake chambers to a value of around 90 psi, this speeds application times in the event of an emergeny and permits a consistent hold-off pressure. (system pressure fluctuates between cut-in and cut-out pressure values.)

128
Q

Because some systems use an inversion valve, what happens, in the event of a primary air system failure?

A

The inversion valve allows for a modulated application of the emergency air brake chambers by relieving air from the hold-off chamber in direct proportion to the application pressure at the treadle valve.

129
Q

What multi-functions are often incorporated in spring brakes?

A

In multifunction valves that contain parking and inversion functions.

130
Q

How can you test the inversion valve?

A

The inversion valve can be tested by draining the primary air tank and making a service brake application.
In a properly operating system, the primary brakes chould apply by means of releasing the air pressure from the parking brake chamber.

131
Q

What is an example of air brake failure originating from one source and make themselves evident in other locations?

A

An example of this would be when a faulty parking brake chamber diaphragm allows allows air to travel through the service side of the brake chamber.
The air then travels through the normally open exhaust port of the service relay valve.
The air leak at the exhaust port of the relay valve is often misdiagnosed as a faulty relay valve, when in actuality the parking brake chamber needs to be replaced.

132
Q

What is a method of isolating whether a concern is coming from the tractor or trailer?

A

To disconnect the trailer (gladhand) air connections from the tractor. If the problem on the tractor discontinues, the original source of the concern is on the trailer.

133
Q

What components make up what is called the foundation brake assembly?

A

The s-cam, or disc brake mechanism itself, linings or pads, and related parts such as brake chamber(s), slack adjuster(s), and parking brake components.

134
Q

What energy does vehicle braking require?

A

All vehicle breaking requires that kinetic energy (the energy of motion) be converted to heat energy by means of friction; the heat energy must then be dissipated to the atmosphere.
The foundation brake assembly sonsists of those brake components that are responsinble for affecting the retarding effort required to stop a vehicle.

135
Q

In a typical S-cam shoe/drum assembly, what do slack adjusters do?

A

Slack adjusters connect the brake actuation chambers with the foundation brake assembly.
Slack adjusters are levers, so their length is critical

136
Q

What does the greater distance between the centerline of the S-cam and the point and the point at which the brake chamber clevis connect with the slack adjuster result it?

A

The greater its leverage.

137
Q

What do Slack adjusters convert?

A

Slack adjusters also convert the linear force produced by the brake chamber into torque (twisting).
Current slack adjusters are required to be automatically adjusting.

138
Q

What will have an effect on the operation of the foundation brake assembly?

A

Slack adjustment and brake chamber installation will have an effect on the operation of the foundation brake assembly.

139
Q

How will you know that the slack adjuster has correct adjustment?

A

When the shortest possible stroke is obtained without any dragging of the brake linings against the brake drum or rotors.

140
Q

Describe what happens when a brake shamber is installed.

A

Whan a brake chamber is installed, the purhrod should be shortened to a length that produces an angle close to 90 degrees when the brakes are fully applied.
This position will leverage the maximum mechanical advantage of the slack adjuster on the S-cam.

141
Q

What are parking and brake performance related to?

A

brake adjustment.

142
Q

What can foundation brake problems cause?

A

unbalanced braking, grabbing, not releasing, or failure to apply.

143
Q

What are some causes of foundation brake problems?

A

Use of improper replacement components, cargo weight overloads, lining contamination, poorly maintained or installed brakes, broken or malfunctioning brake components, brakes out of adjustment, and overheating.

144
Q

What is necessary to service a spring brake chamber?

A

To service a spring brake chamber, it is necessary to “cage” the main spring before any service is performed.

145
Q

How would you normally manually cage a spring brake?

A

A spring brake is normally caged by tightening a nut on a supplied release tool. Some release tools have a pair of lugs that engage to an internal cage plate. When the cage nuty is tightened, the cage plate compresses the main spring.

146
Q

If a clevis is not positioned correctly onto the pushrod, what may happen?

A

The brake may not completely release.
The clevis has a tapped yoke bore for a pushrod to engage.
Most manufacturers provide slack adjuster installation templated facilitating correct installation.
Some brake chambers have welded yokes and are not adjustable.

147
Q

If equipped with a quick-attach clevis, what is considered the maximum allowable before replacement?

A

A 0.060 inch (1.52mm) clearance between the clevis pin and the clevis collar.
Replace the clevis if the collar hole is worn beyond this limit.

148
Q

What does an automatic slack adjuster do.

A

An automatic slack adjuster automatically adjusts the clearance between the brake linings and the brake drum or rotor.
Automatic slack adjusters control clearance by sensing increases in either stroke length or lining to drum or rotor clearance, so no routing manual adjustment is required.

149
Q

What are common causes of sluggish service braking in an air system?

A

Common causes are problems concerning foundation brake geometry.
Problems such as an obstruction in a brake chamber or poor alignment of the brake linkage can product apply-and release-rate performance problems.

150
Q

What happens when the park control valve closes?

A

Air in the hold-off section of the spring chamber is exhausted, allowing the main spring to apply the parking brake.

151
Q

How can you check for leaks in air chambers?

A

Air chambers can be checked for leaks using a soapy water spray.

152
Q

Where do you replace failed diaphragms?

A

Failed diaphragms can be replaced in the service chamber.

153
Q

How would you check return springs?

A

By watching for full retraction of the service brake chamber push-rod when the air is released from the chamber.

154
Q

What should pushrods not contact?

A

Rushrods should not contact any part of the housing or mounts during extension or retraction.

155
Q

What should you check mounting brackets for?

A

Mounting brackets should be checked for cracks.

A cracked mounting bracket can be caused by incorrect application, compounding brakes, or abuse, and should be replaced.

156
Q

The slack adjuster critical in maintaining _____?

A

Required free-play and actuation angel.

Current slack adjusters are required to be automatically adjusting, but their operation must be verified routinely.

157
Q

What does the slack adjusters connect on each wheel?

A

Slack adjusters connect the brake chambers with the foundation assemblies on each wheel.

158
Q

How do slack adjusters connect to the pushrod of the brake chamber?

A

Slack adjusters connect to the pushrod of the brake chamber by means of a clevis yoke (threaded to the pushrod) and pin.

159
Q

What are slack adjusters mounted to?

A

Slack adjusters are spline-mounted to the S-cam and positioned by shims and an external snap ring.

160
Q

Into what does the slack adjuster convert the linear force of the brake chamber rod?

A

It coverts it into rotary force or torque and multiplies it.

161
Q

What does the distance between the slack adjuster S-cam and the clevis pin axis define??”

A

It defines the leverage factor - the greater this distance, the greater the leverage.

162
Q

What is the objective of brake adjustment?

A

To maintain a specified drum-to-lining clearance and a specified amount of free stroke.

163
Q

What is “free stroke?”

A

Free stroke is the amount of slack adjuster stroke that occurs before the linings contact the drum.

164
Q

How are sladk adjusters serviced?

A

Slack adjusters are serviced by lubricating with grease or automatic lubing systems

165
Q

How are the seals in slack adjuster tubes always installed?

A

The seals in slack adjuster tubes are always installed with lip angle facing outward.

166
Q

What should happen when grease is pumped into the slack adjuster?

A

Grease should exit the seal lip when the internal lubrication circuit has ben charged.

167
Q

When should automatic slack adjusters not need adjusting?

A

Automatic slack adjusters not need adjusting during regular inspection.
If they do, it is an indication that they are not functioning correctly, and the truck should be taken out of service and repaired.

168
Q

How do yo adjust manual slack adjusters?

A

Manual slack adjusters are adjusted by rotating the adjusting screw to set the specified freeplay.

169
Q

How should a wheel-up adjustment be performed?

A

The freeplay is set to minimum drag of the shoe/drum relationship.

170
Q

How do you replace a failed slack adjuster?

A

When replacing a failed slack adjuster, the distance between the axis of the S-cam bore and the clevis pin bore must be maintained.
A differene of one-half inch can greatly alter the brake torque and unbalance the brake.

171
Q

How would you check slack adjusters?

A

Slack adjusters can be checked by backing off the adjustment and making multiple brake applications while watching for the brakes to self-adjust.

172
Q

When performing foundation brake servicing, what should also be done?

A

All of the hardware mounted on the axle spider should be inspected and replaced if required.

173
Q

What must the radial play of the S-camshaft typically not exceed?

A

Typically must not exceed 0.030 inches (0.75 mm).

174
Q

What should the axial end-play typically not exceed?

A

Axial end-lay should not exceed 0.60 inches (1.52 mm)

175
Q

What should rollers and cams be inspected for?

A

For flat spots and twisted splines caused by compounded brake applications.

176
Q

Why should you never mix s-cams?

A

Because the S-cam profile is reversed from side to side.

177
Q

What spring steel components should be replaced?

A

Retraction springs and snap rings.

178
Q

What should you do if you have frozen anchor pins?

A

Anchor pins should be free to move in the anchor pin bore. If they are frozen, the anchor pin and bushing should be replaced.

179
Q

How do you check spider fasteners?

A

Spider fastener integrity should be checked at each brake job. Spiders should be inspected for cracks at each brake job.

180
Q

Why should S-cam neve be lubricated?

A

S-cam profiles depend on friction and should never be lubricated.

181
Q

What conditiion should anchor pins be when checked.

A

Anchor pins should be smooth and should move freely in their bore.

182
Q

When are retainers typically replace?

A

Typically, retainers are replaced when the other brake hardware is replaced.

183
Q

How are air-actuated heavy-duty truck disc brakes similar to automobile hydraulically actuated disc brakes?

A

Air disc brakes use an air chamber and pushrod to apply braking torque.

184
Q

What happens within the caliper assembly?

A

Pistons transmit the clamping force to brake pads, which affect retarding effort by converting the kinetic energy (energy in motion) to friction and then heat.

185
Q

What is usually an indication of caliper assembly seizure?

A

Short outboard friction pad service life is usually an indication of caliper assembly seizure - that is, the sliding action of the brake caliper ceases.
The caliper sliide pins are usually at fault.

186
Q

which brakes typically offer shorter stopping distance?

A

Air disc brakes typically offer shorer stopping distance than drum brakes.

187
Q

What is truck brake shoes?

A

Truck brake shoes are, in most cases, fixed anchor assemblies mounted to the axle spider and actuated by an S-camshaft.

188
Q

How are friction facings or linings mounted to brake shoes for a heavy-duty truck ?

A

Brake shoes for a havy-duty truck, can have their friction facings or linings mounted to the shoe by riveting or by fasteners.

189
Q

In most current applications, the shoes are remanufactured and replaced as an assembly - that is, with the new friction facing alfreay installed. What should you do when the linings on a single wheel are damaged, such as in the event of wet seal axle lube falure?

A

The linings of both wheel assemblies on the axle should be replaced to maintain brake balance.

190
Q

How should the friction pads in air disc brake assemblies be changed?

A

The friction pads in air disc brake assemblies should be changed in paired sets- that is both wheels on an axle.
Replacement foundation brake components must be the same as the original components.
Never replace a foundation brake component with a part that is not equivalent to the original component unless instructed to do so in the truck or brake component manufacturer’s service manual or service bulletin.

191
Q

What happens if foundation brake components are replace with parts that are not the same as the original components?

A

Serious braking defects may occur, resulting in increased stopping distancs or wheel lockup.

192
Q

what should the technician do when replacing brake shoes or pads?

A

The technician should refer to the friction code stamped on the lining material or review the original which specifications to ensure the correct lining is installed.

193
Q

What happens when incorrect linings, either too aggressive or not aggressive enough for the application?

A

Can result in premature brake wear or failure of the truck to stop in the required distance.

194
Q

What happens if a truck has been changed from one application to another - for example, a line-haul truck has been modified to become a dump truck?

A

It is possible that the brake lining friction material would also need to be changed to reflect the new vehicle application.

195
Q

If brake drums are to be reused, what should they be inspected for?

A

They should be inspected for size specification, heat discoloration, scoring, glazing, threading, concaving, convening, bellmouthing, and heat checking.

196
Q

Why can’t used truck brake drums be successfully machined?

A

Used brake drumss can seldom be successfully machined due to heat tempering.

197
Q

What is the maximum legal service limit for most truck and trailer brake drums?

A

The maximum legal service limit for most truck and trailer brake drums is 0.120 inches (3.05 mm)
The machining limit is 0.60 inches (1052 mm).
The discard limit is 0.090 inches (2.29 mm)

198
Q

What would you use to check brake drum diameter?

A

WA drum micrometer.

199
Q

What should you do when inspecting b

Frake motors?

A

Brake motors should be visually inspected for heat checking, scoring, and cracks.
Rotors must be measured for thickness with a micrometer.
A dial indicator is used to check rotor run-out and parallelism.
In highway applications, rotors tend to outlast drums and are often reused after a brake job.
They must be turned within legal service specifications using a heavy-duty rotor lathe.

200
Q

When should wheel bearings be inspected?

A

Wheel bearings should be inspected at each brake job and at each preventive maintenance (PM) service that requires that the wheels be removed.

201
Q

What kind of bearings do most current trucks use?

A

Most current trucks use wet bearings - they are lubricated with liquid lubricant, usually gear lube.

202
Q

Since the use of tapered roller become almost universal, what does a tapered wheel bearing assembly consist of?

A

A tapered wheel bearing assembly consists of a cone assembly and a cup or race.

203
Q

What does the cone assembly consist of?

A

Tapered of rollers mounted on a roller cage.

204
Q

What happens if the race fits loose in the hub?

A

The bearing race or cup is interference-fit to the hub. If the race fits loose in the hub, both the race and hub must be replaced.

205
Q

What supports the load in each wheel hub?

A

A pair of tapered roller bearings supports the load in each wheel hub.

206
Q

Are the cones and cups of tapered roller bearings interchangeable?

A

No. When damage is evident in either the cup or the cone both should be replaced.

207
Q

How should you clean the bearings, races, and cups?

A

Bearings, races, and cups should be cleaned with solvent and air dried, ensuring that the cone is not spun out by the compressed air.

208
Q

How would you inspect the cone rollers and the cone?

A

Both the cone rollers and the cone should be inspected for spalling, galling, scoring, heat discolortion, and any sign of hard surface failure.

209
Q

What should you do when reinstalling wet bearings?

A

Make sure they are pre-lubed with the same with oil to be used in the axle hub.

210
Q

What would you do when grease packed bearings are used.

A

When grease-packed bearings are used, the bearing cone must be packed with grease.
This procedure may be performed by hand or by using a grease gun and cone packer.

211
Q

What does grease-lubricated wheel bearings require?

A

Grease-lubricated wheel bearings require the use of a high temperature axle grease.
It is a bad practice to pack wet bearings with axle grease, and it may shorten bearing life by reducing lubrication efficiency

212
Q

What is the primary cause of bearings failure?

A

Dirt contamination. This may be due to the condition of a vehicle is operated under or poor service practice.

213
Q

What else can cause bearing failure?

A

Lubrication failures caused by an inappropriate lubricant or lack of lubricant (caused by a wheel seal) also account for al large number of bearing failures.
Lack of lubricant will result in rapid failure and a bearing welded to an axle.

214
Q

Many bearing failures are caused by brinelling. What is brinelling?

A

Brinelling appears as indentations across the bearing race.

Brinelling can occur when the bearing is not rotating and receives a sudden impact, or when subjected to vibration.

215
Q

What will incorrectly adjusted bearings result in?

A

Incorrectly adjusted bearings can fail rapidly, especially when the preload is high.
The result of high preload on a bearing can cause the bearing to friction-weld to the axle.
Overload bearings fail over time rather than rapidly.

216
Q

What is the correct method of adjusting a tapered roller wheel bearing as outlined by the Truck Maintenance Council (TMC) division of the American Trucking Association.?

A
  1. The TMC bearing adjustment method requires that the bearing be seated to a specified torque value at the adjusting nut while rotating the wheel bearing.
  2. This part of the procedure is designed to seat the bearing with a preload.
  3. Next, the adjusting nut should be backed off as specified.as specified.
  4. On a two-nut that should be torqued to specification.
  5. On a two-nut system the bearing setting is locked in place on the axle spindle by a lock or jam nut that should be torrqued to specification.
  6. On other axles, a single nut may be used. This single nut is adjusted, then a lock is installed.
  7. T must be measured ahe lock can be a locking spring, locking tabs, or a cotter pin lock.
  8. In all designs the technician must make sure that the lock is secured after the bearings are installed and adjusted.
  9. Finally, the bearing end-play must be measured with a dial indicator: The required specification is between 0.001 inches (0.0254 mm) and 0.005 inches (0.127 mm).
  10. End play must be present. If there is less than 0.001 inches of end-play, the adjustment procedure should b..e repeated.
217
Q

What is responsible for supporting the vehicle weight?

A

Wheel bearings are responsible for supporting the vehicle weight while allowing wheel rotation.

218
Q

What does proper adjustment for wheel bearings require?

A

Proper adjustment requires some end-play or clearance for lubrication and bearing expansion at operating temperatures.
The adjustment and proper lubrication is crucial for these bearings to operate without damage or seizure from excessive heat buildup.

219
Q

Some trucks now have hubs that use bearing assemblies that produce preset clearances when installed according to specifications. How are these hubs checked?

A

These hubs are checked for end play like a conventional bearing assembly.
The typical installation has the technician install the assembly on the spindle and tighten the training nut to a specified torque value.
The torque value can be very high, sometimes as much as 600 ft/lbs.
After installation, the technician should check that the wheel end rolls freely and use a dial indicator to check wheel bearing end-play.
If the wheel bearing assembly has failed, typically the entire assembly (hub, bearings, and seal) is replaced as a unit.

220
Q

What does FMVSS 121 require?

A

Requires that all trucks be equipped with a mechanical parking and emergency brake system.

221
Q

How is the parking brake system different from the service brake system?

A

The parking brake system is generally a separate system from the service brake system, with its own lines, chambers, and valves.

222
Q

How is the mechanical braking force obtained?

A

By springs located in spring brake chambers.

The use air acting on a hold-off diaphragm to release the spring brake.

223
Q

What does the spring in a spring brake chamber require to fully release?

A

The spring in a spring spring brake chamber requires approximately 60 psi acting on its hold-off chamber to fully release.
It is therefore capable of applying the equivalent of a 60 psi air application when there is no air acting on the hold-off chamber.

224
Q

What is a spring brake chamber and how does it work?

A

Spring brake chambers are dual chamber assemblies that combine a service application chamber in the front section and spring a chamber in the rear section.

225
Q

What does the size rating such as 30/30, 24/24, 30/24 refer to?

A

Refers to the surface area of the service and hold-off diaphragms.

226
Q

What happens if the plastic seal is not in place in the caging port at the rear of the spring diaphragm?

A

If the seal is not in place, the internal components can rapidly corrode.

227
Q

What causes a spring brake chamber assembly to fail?

A

Spring brake chamber assemblies fail when either the service or hold-off diaphragms puncture or when the spring breaks.

228
Q

What indicates spring breakage?

A

It’s often indicated by an air leak at the hold-off diaphragm that occurs when the fractured spring punctures it.

229
Q

How should spring brakes be handled with caution, even when it is known that the main spring has fractured?

A

It is recommended that the power spring be cut prior to disposing of the chamber.
The chamber can be installed in a specially designed box and a torch used to safely cut the power spring.

230
Q

What does air leaking from the service relay valve indicate when the parking brakes are released?

A

That the center seal in the parking brake chamber is leaking.

231
Q

How would you safely replace a spring brake chamber?

A

Safety should be the first concern of the technician.
If possible, cage the power spring.
If this is not possible, care should be taken to stay out of its direct path.

232
Q

What could happen if the chamber is corroded or otherwise damaged?

A

There is a possibility of the spring coming loose from the housing.

233
Q

How would you remove the parking brake chamber?

A

Remove the clevis pin(s) and remove the two nuts holding the chamber to the chamber bracket.
The chamber bracket should be thoroughly inspected for cracks.

234
Q

How would you install the new chamber?

A

When installing the new chamber, the pushrod should be cut to the correct length following manufacturer’s procedure.
If the replacement chamber is the welded clevis type, it will not be necessary to cut the pushrod.

235
Q

How would you safely discard spring brakes?

A

It is unsafe to discard spring brakes without first disarming them.
Disarming a spring brake means releasing the energy stored in the spring.
This is achieved by placing the entire spring brake assembly in a disarmament chamber and, using a torch, cut the spring brake chamber clamps and separate the chamber.
The entire assembly should remain in the disarmament chamber until the main spring can be observed to be free.

236
Q

Why is the supply of air to the parking brake circuit pressure-protected?

A

protected so that in the event of a complete primary or secondary circuit failure, there is sufficient air for a complete apply/release sequence.

237
Q

What is the result of restrictions in the lines, hoses and fittings supplying the spring brake chambers?

A

Can slow both application and release times.

238
Q

How would you verify spring brake valve operation?

A

Operation can be verified with an air pressure gauge.

Some spring brake valves can be rebuilt, however, they are normally replaced as a unit.

239
Q

In what are spring brake valves often incorporated?

A

Often incorporated in multi-function valves.
It is critically important that when valves malfunction, they be replaced by matching the part numbers, remembering the two valves that appear identical may have widely different performance characteristics.

240
Q

What does the parking brake system of a tractor-trailer combination manage?

A

Manages hold-off pressure delivered to the spring brake chambers.
Two and three-dash valve systems are used.

241
Q

The FMVSS 121 requires that the shape of each valve conform to what? (the color of each valve is an industry standard).

A

Yellow diamond/square: the system spring brake (park) control valve.
This valve masters the sparking brakes on a tractor-trailer combination.
It is pushed inward to release the parking brakes.
When pulled out to its park position, it cuts the air supply to the trailer (thus applying its park/emergency brakes) and puts the tractor into park/emergency mode.
Red octagonal knob: the trailer supply valve.
The system spring brake control valve must be pushed in and actuated before the trailer supply valve will work.
The valve supplies system air to the trailer(s) and must be ON to release the trailer spring brakes.

242
Q

How do you test a parking brake dash control valve?

A

To test a parking brake dash control valve, first build system air pressure to governor cut-out and shut off the truck.
Next, chock the vehicle with appropriate wheel chocks and push the control vale in to release the parking brakes.
Observe that the brakes fully release and then make several service brake applications.
When system air pressure drops to between 20 and 40 psi, the parking brake knob should pop.
Spring brake valves can be rebuilt with the correct service kit.

243
Q

Spring brake chambers are usually equipped with a cage bolt. What is a common style cage bolt?

A

A common style cage bolt is threaded and has a pair of lugs that engage to an internal cage plate.

244
Q

What occurs when a nut i turned down the cage bolt?

A

When a nut is turned down the cage bolt, the cage plate is pulled inward, compressing the main spring of the spring brake chamber.
This releases the parking brake.

245
Q

If the vehicle on-board air supply is available, how may a spring brake be caged?

A

A spring brake may be caged by chocking the wheels and releasing the parking brakes (supplying air to the hold-off chambers) and then installing the cage bolt to the cage plate with the main spring already compressed.
It is important to ensure that the lugs in the cage bolt are properly engaged in the cage plate.

246
Q

What are cage plates are manufactured from?

A

Cage plates are manufactured from aluminum alloy, and those in older spring brake assemblies were susceptible to corrosion.
Cage spring brake assemblies should always be handled with a great amount of care.
Spring brake chambers should always be removed and installed fully caged.

247
Q

What is current practice of failed hold-off diaphragms?

A

When spring brake hold-off diaphragms fail, a piggyback assembly (spring brake assembly minus the service chamber) or entire spring chamber is used to repair the condition.
The spring brake chamber band clamps should never be removed with the main spring in un-caged condition.
Many current spring brake assemblies have chamber clamps that cannot be unbolted.

ASE Brakes (3 decks)

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