Frame and Chassis

Question 1

How would you check the air parking brake?

Answer 1

Start the engine and wait until the air system reaches system pressure
Puh in the yellow-colored diamond-shaped dash control valve knob to release the park brakes.
It should stay in; if it doesn’t, check for a pressure loss or a defective control valve.
Test the park brakes by putting the truck in low gear, moving it forward at full speed, and applying the park brakes by pulling out on the knobs.
The truck should stop promptly.

Question 2

What affects the park brake performance?

Answer 2

Park brake performance is affected by brake free stroke adjustment. An excessive free stroke will reduce both park and service brake performance.

Question 3

What does the governor manage?

Answer 3

Compressor loaded and unloaded cycles. It defines the system pressure.

Question 4

What values is system pressure in most trucks set at?

Answer 4

System pressure in most trucks is set at values between

120 and 135 pSI, with 125 psi being most typical.

Question 5

What is cut out pressure?

Answer 5

Cur out pressure is the pressure at which the governor outputs the unloader signal to the compressor.
The unloader signal is maintained until the pressure in the supply tank drops to the cut-in valve.

Question 6

What is one way to easily check the governor’s operation?

Answer 6

One method is to drop the aiW top of the governor, release the lock-nut and turn the adjusting screw either clockwise to lower or counterclockwise to raise the cut-out pressure.

Question 7

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

Answer 7

Will result in high system pressures.

Question 8

What is usually an indication of governor or compressor unloader malfunction?

Answer 8

If the safety pop-off valve on the supply tank trips (this ours at 150 psi)/

Question 9

What happens when the compressor unloader cycles?

Answer 9

It triggers the purge valve in the air dryer sump, ejecting moisture and any accumulated sludge.
The purge valve may become clogged and need to be cleaned.

Question 10

What temperature should the air drier heater operate?

Answer 10

The heater should operate when the temp falls below about 40 degrees F.

Question 11

How should you test the purge valve?

Answer 11

By cycling the compressor through several cut-in/cut-out cycles, and observing the operation of the purge valve. Each time the unloader cycles, there should be a discharge of air and moisture from the bottom of the air dryer.

Question 12

How do you check the air system for leaks brakes applied?

Answer 12

1. First block the wheels on the vehicle.
2. Build air pressure to governor cut-out, release the parking brakes and shut off the engine.
3. Walk around the unit, listening for audible air leaks.
4. Then check dash gauges for system pressure loss.,
5. Investigate any out-of-specification loss.

Question 13

According to the Commercial Vehicle Safety Alliance, under what conditions shall a vehicle be declared out of service?

Answer 13

A vehicle with more than 20% of its air brakes defective (measured with the engine off, reservoir pressure at 80-90 psi, and the brakes fully applied.

Question 14

How do you check the air system for leaks brakes released?

Answer 14

1. Begin by starting the engine and bringing the air supply to system pressure.
   2, With the engine idling, make and hold a full brake application until pressure stabilizes.
   If the compressor is unable to maintain reservoir pressure of 89-90 psi with the engine at idle and the brakes fully applied, it is recommended that the truck be deadlined until repairs are completed.

Question 15

How do you examine the truck for audile air leaks?

Answer 15

Shut off the engine, and have an assistant hold the brakes applied while you examine the truck for audible leaks.

Question 16

What part of the brake system does the supply tank feed?

Answer 16

The primary and secondary reservoirs of the brake system.

Each is pressure protected by means of a one-way check valve.

Question 17

How do you verify one-way check valve operation?

Answer 17

Drain the supply tank and watch for pressure loss in the other reservoirs.

Question 18

How are one-way check valves used throughout an air-brake system?

Answer 18

To pressure-protect and isolate portions of the circuit.

Question 19

What does two-way (double) check valves do?

Answer 19

Two-way check valves provide a means of providing the primary circuit with secondary circuit air and vice versa, in case of a circuit failure.
Is a T with two inlets and a single outlet.
It outputs the larger of the two-source pressures to the outlet ports and blocks the lower pressure source; the valve will shuttle in case of a change in the source pressure value.

Question 20

What should the technician do before performing a pressure drop test on a heavy truck?

Answer 20

Ensure the engine is stopped and the system is at full pressure.

Question 21

How do you perform a pressure drop test on a heavy truck?

Answer 21

Make and maintain a brake application.

Allow pressure to stabilize for one minute and then begin timing for two minutes while watching the dash gauge.

Question 22

What is the pressure drop for single vehicles?

Answer 22

4 psi within two minutes while watching the dash gauge.

Question 23

What is the pressure-drop for singles’ combination vehicles?

Answer 23

6 PSI within two minutes.

Question 24

What federal legislation defines system air buildup times?

Answer 24

FMVSS 121

Question 25

What does a common check performed by enforcement agencies require?

Answer 25

The supply circuit on a vehicle be capable of raising air system pressure from 85 to 100 psi in 2 seconds or less.
Failure to achieve this buildup time indicates:
a worn compressor
a defective compressor unloader assembly
supply circuit leakage,
or a defective governor.

Question 26

What does FMVSS 121 require when the system pressure drops below 60 psi

Answer 26

FMVSS 121 requires that a driver receive a visible alert, in most cases this is accompanied by an audible alert, usually buzzer.
A low air pressure warning device is fitted to both the primary and secondary circuits..
The switch is electrically closed whenever the air pressure being monitored is below 60 psi. When the air pressure exceeds 60 psi, the switch opens.

Question 27

How can you verify the operation of a low air pressure warning switch?

Answer 27

This can be done by pumping the service application valve until the system pressure drops to the trigger value.

Question 28

How would you check for the proper functioning of the inversion (spring brake control) valve?

Answer 28

The air system should be fully charged (Min. 100 psi), and the spring brakes must be released.
Next, locate and drain the wet tank and then the primary axle service reservoir completely by means of the draincocks.
The primary gauge needle will drop to 0 psi, indicating a pressure loss for that system, which is normal.
The secondary gauge needle must remain at system pressure (min. 100 psi) to start the check of the inversion valve.
Depress the foot valve in a normal manner.
The spring brakes on the rear axle (s) will apply as the air is exhausted from the brake chambers.
Release the foot valve, and the spring brakes do not or only partially release, then the inversion valve has failed and should be replaced.

Question 29

What does the tractor protection valve do?

Answer 29

The tractor protection valve protects the tractor air supply under a trailer breakaway condition or severe ait leakage.

Question 30

Which two air lines connect a tractor brake system with a trailer air circuit?

Answer 30

The trailer supply line supplies the trailer with air for braking and any other pneumatic systems such as its suspension.
The trailer service line is the service brake signal line.
Both these lines are plumbed to the tractor protection valve.
When the trailer supply dash valve (emergency) red hexagonal is in its off position the tractor protection valve is also off.

Question 31

What happens when the driver makes a service brake application?

Answer 31

No air will exit the tractor protection valve service signal line to the trailer.

Question 32

What happens when the trailer air supply valve is open?

Answer 32

Air will open the tractor protection valve and supply the trailer pneumatic system with air.
Then when the driver makes a service brake application, the service brake signal air will be transmitted through the tractor protection valve to apply its service brakes.

Question 33

The trailer air supply controls the trailer park brakes. So what happens whenever air supply is interrupted?

Answer 33

When the trailer is being parked or unintentionally in a breakaway, the spring brakes will apply regardless of how much pressure is in the trailer air reservoirs

Question 34

From what is the tractor protection valve designed to isolate?

Answer 34

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.
These pressures, the spring brakes on both the tractor and trailer would be partially applied.

Question 35

Whatis a common check performed by law enforcement agencies in regards to the supply circuit on a vehicle?

Answer 35

Law enforcement agencies requires that the supply circuit on a vehicle be capable of raising air system pressure from 85 to 100 psi in 25 seconds or less.

Question 36

What could be the problem if the supply circuit on a vehicle fails to raise air system pressure from 85 to 100 psi in 25 seconds or less?

Answer 36

Failure to achieve the build-up time indicates a worn compressor, defective compressor unloader assembly, supply circuit leakage, or a defective governor.

Question 37

What is the trailer control valve used for?

Answer 37

To actuate the trailer service brakes on a trailer independently of the tractor service brakes. This valve is also known by the terms: trolley valve, broker brake, and spike.

Question 38

What is the source of air supplied to the trailer control valve?

Answer 38

It is usually the secondary circuit. Brake application pressure air is modulated proportionally with control valve travel.

Question 39

What are two-way check valves used for and where are they located?

Answer 39

Two-way check valves are used in the application circuit of the trailer brakes. They are located downstream from both the trailer control valve and the treadle valve.

Question 40

In a typical system, what sourced air does the trailer application brake valve (trolley or spike) use, and what sourced air does the treadle valve trailer service signal use, to activate the trailer service brakes?

Answer 40

In a typical system, the trailer application bake valve uses secondary circuit sourced air and the treadle valve trailer service signal uses primary circuit sourced air to activate the trailer service brakes.

Question 41

If a driver was using the spike to bring the vehicle to a stop and during this process, an emergency occurred that required a panic application of the treadle valve, , what will happen?

Answer 41

The two-way check valve would permit the source air to change from secondary circuit air from the trailer control valve to the higher source pressure value from the treadle valve.

Question 42

What would you use to verify the trailer control valve performance?

Answer 42

An accurate air pressure gauge. The trailer is supplied with air by means of air hoses.
Service and trailer supply air are sent to the tractor protection valve.

Question 43

How would you perform the antilock brake system (ABS) operational system self-test?

Answer 43

All current ABSs are electronically managed.
a typical ABS adapts a standard air or hydraulic system for electronic management. All current systems are designed to default to non-managed operation in the event of an electrical or electronic malfunction. (New trucks are equipped with ABS brakes and are mandatory.

Question 44

What does ab ABS consist of?

Answer 44

An ABS consists of a means of monitoring the speed of each wheel, an electronic control unit (ECU) or ABS module to manage the system, and a modulator assemble to affect the outcomes of the ECU logic processing by modulating the pressures to each wheel.

Question 45

ABS brakes are especially effective at managing split-coefficient braking conditions. When does split coefficient braking occur?

Answer 45

Split coefficient occurs when road surface conditions (icy, wet, gravel, dry, etc) differ from one wheel to another.

Question 46

What are all ABSs equipped with and how can these be accessed?

Answer 46

Self-diagnostics. These can be accessed by flash or blink codes using the dash warning light or a scan tool (digital diagnostic reader) connected either at the ATA connector in the cab or directly to the ABS module.

Question 47

What does the self-test consist of when an electronic ABS is powered up.

Answer 47

The self-test consists of testing the signal range in the wheel speed sensors and cycling the solenoids in the system actuators.

Question 48

How would you access the ABS self-diagnostics?

Answer 48

Access the ABS self-diagnostics by selecting the correct message identifier (MID).
Identify the failure mode indicator (FMI) that has triggered the fault code and repair the failure using the OEM service instructions.

Question 49

How would you inspect coupling air lines, holders, and gladhands?

Answer 49

1. Visually check all hoses and lines in the brake system.
2. Service and trailer supply air is plumbed from the tractor protection valve.
3. This air is transferred to the trailer by air hoses with couplers known as glad-hands. Gladhands enable coupling between the tractor air supply and the trailer pneumatic circuit.
   Gladhand seals are usually manufactured out of rubber.
   They are retained in an annular groove in the gladhand and are easily replaced when they fail.

Question 50

What are some of the deficiencies that should be noted during an annual inspection of the brake chambers, and air lines?

Answer 50

1. Absence of braking action on any axle, required to have brakes, upon application of the service brakes.
2. Missing or broken mechanical components including shoes, lining pads, spring, anchor pins, spiders, cam rollers, pushrods, and air chambers mounting bolts.
3. Loose brake components including air chambers, spiders, and camshaft support brackets.
4. Audible air leak at brake chamber (ruptured diaphragm, loose chamber clamp, etc.
5. Hose with any damage extending through the outer reinforcement ply. Rubber impregnated fabric cover is not a reinforcement ply. Thermoplastic nylon may have braid reinforcement or color difference between cover and inner tube, Exposure of second is cause for rejection.
6. Bulge or swelling when air pressure is applied.
7. Any audible leaks from hose or tubing.
8. Two hoses improperly joined (such as a splice made by sliding the hose ends over a piece of tubing and clamping the hose to the tube.)
9. Air hose cracked broken or crimped.
10. Tubing cracked, damaged by heat, broken or crimped.

Question 51

What are the conditions of brake linings that cause them to need to be replaced?

Answer 51

1. The lining or pad is not firmly attached to the shoe.
2. Saturated with oil, grease or brake fluid.
3. Non-steering axles: Lining with a thickness less than 1/4 inch at the shoe center for air drum, brakes, 1/16 inch or less at the shoe center for hydraulic and electric drum brakes, and less than 1/8 inches for air disc brakes.
4. Steering axles: Lining with a thickness less than 1/4 inch at the shoe center for drum brakes, less than 1/8 inch for air disc brakes, and 1/16 inch or less for hydraulic disc and electric brakes.
   Unequal lining wear should be noted and the cause investigated.

Question 52

How are truck brake shoes mounted and actuated?

Answer 52

Truck brake shoes are in most cases fixed-anchor assemblies mounted to the axle spider and actuated by an S-camshaft.
Brake shoes for a heavy-duty truck can have their friction facings or linings mounted to the shoe by bonding, riveting, or by screw fasteners.

Question 53

How are current application shoes remanufactured and replaced?

Answer 53

Current application shoes are remanufactured and replaced as an assembly, that is, with the new friction facing already installed.

Question 54

What should you check for when reusing shoes?

Answer 54

Check for ar deformities usually caused by prolonged operation without brake adjustment.

Question 55

What should you do when reconditioning shoes with bolted linings?

Answer 55

When reconditioning shoes with bolted linings, if the original fasteners are reused, the lock washers should at least be replaced.
Riveted linings should be rivet in the manufacturer’s recommended sequence.

Question 56

How is the friction rating of linings coded?

Answer 56

By letter codes.
Combination lining sets of shoes are occasionally used; these use different friction ratings on the primary and secondary shoes.
When combination friction lining sets are used, care should be taken to install the lining blocks in the correct locations on the brake shoes.

Question 57

What is a good PMI practice for a tandem drive axle truck?

Answer 57

Replace all of the brake linings on a PMI schedule.

Question 58

When the linings on a single wheel are damaged, as in the case of wet seal axle lube failure, what should you do?

Answer 58

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

Question 59

How should you change the friction pads in air disc brake assemblies?

Answer 59

The friction pads in air disc brake assemblies should be changed in paired sets, that is, both wheels on an axle.
Vented disc brakes may have thicker inner pads than outer pads while solid disc brakes usually have equal thickness inner and outer pads.
Heat transfers more uniformly in the solid disc assemblies than vented ones, requiring thicker inboard pads.

Question 60

What are the different factors that brake drum condition can vary?

Answer 60

Including, but not limited to, scoring, bell-mouth condition, concave-like, convex-like, threaded appearance, and hard spots.

Question 61

What does a convex condition indicate?

Answer 61

Indicates a drum diameter greater at the friction surface edges as compared to the center of the friction surface.

Question 62

What does a concave condition indicate?

Answer 62

Indicates a drum diameter greater at the center of the friction surfaces as compared to the edges.

Question 63

When does a bell-mouthed condition occur?

Answer 63

This occurs when the drum diameter is greater at the inboard edge of the drum (next to the backing plate) compared to the outboard edge.

Question 64

What is an out-of-round condition?

Answer 64

This is a variation in the drum diameter at measurements taken 180 degrees apart.

Question 65

How do you check for lateral run out?

Answer 65

A dial indicator is usually mounted on the hub and the face of the rotor is indicated. Total indicated run out should generally not be more than 0.005 inch (0.13 mm)

Question 66

If brake drums are to be reused, what should should you inspect for?

Answer 66

Reused brake drums should be inspected for wear, heat discoloration, glazing, threading, concaving, convexing, bell mouthing, and heat checking.

Question 67

When is it a good practice to machine new drums?

Answer 67

Before installation due to warpage caused by incorrect storage practices after manufacture.

Question 68

What is the maximum legal service limit for 14-inch-diameter brake drums?
What is the machining limit?
What is the discard limit?

Answer 68

The maximum legal service limit for 14-inch-diameter brake drums is 14.120 inches.
The machining limit is 14.060 inches.
The discard limit is 14.090 inches.

Question 69

What must brake rotors be visually inspected for?

Answer 69

For heat checking, scoring, and cracks.

Question 70

How do you measure rotors?

Answer 70

Measure rotors for thickness with a micrometer.
Use a dial indicator to check the 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.

Question 71

What are slack adjusters critical for?

Answer 71

Maintaining the required free play and adjustment angle.

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

Question 72

Since manually adjusting automatic slack adjusters is dangerous and should not be done, when can it be done?

Answer 72

During installation or in an emergency to move the vehicle to a repair facility, because manual adjustment of this brake component:
1, fails to address the true reasons why the brakes are not maintaining adjustment, giving the operator a false sense of security about the effectiveness of the brakes, which are likely to go out of adjustment again soon. and
2. causes abnormal wear to the internal adjusting mechanism for most automatic slack adjusters, which may lead to failure of this brake component.

Question 73

What do slack adjusters connect?

Answer 73

Slack adjusters connect the brake chambers with the foundation assemblies on each wheel. They are connected to the pushrod of the brake chamber by means of a clevis yoke (threaded to the pin. purhrod_andpinn

Question 74

How are slack adjusters mounted?

Answer 74

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

Question 75

What does the slack adjuster convert the linear force of the rod into?

Answer 75

The slack adjuster converts the linear force of the brake chambers rod into rotary force or torque and multiplies it.

Question 76

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

Answer 76

The leverage factor; The greater this distance, the greater the leverage.

Question 77

What is the objective of brake adjustments, whether manual or automatic?

Answer 77

Is to maintain a specified drum to lining clearance and a specified amount of Freeplay.

Question 78

What is Freeplay?

Answer 78

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

Question 79

What lubricates slack adjusters?

Answer 79

Grease or automatic lubing systems lubricate slack adjusters.

Question 80

How are the seals in slack adjusters always installed?

Answer 80

The seals in slack adjusters are always installed with the lip angle facing outward.
When grease is pumped into the slack adjuster, grease will easily exit the seal lip when the internal lubrication circuit has been charged.

Question 81

What does failure to adjust the slack adjuster clevis with the proper template cause?

Answer 81

Improper self-adjusting action in the slack adjuster.
If necessary, adjust the clevis on the end of the pushrod until the outer hole in the clevis is completely visible through the template opening.

Question 82

What should you do when performing foundation brake servicing?

Answer 82

All of the hardware mounted on the axle spider should be inspected and replaced if required.
There should be no radial play of the S-camshaft
Rollers and cams should be inspected for flat spots, and it is good practice to replace any spring-steel components such as retraction springs and snap rings.

Question 83

Why should you install S-cam bushing seals?

Answer 83

So that the spider outboard bushing sealing lip faces inward toward the bushing and the inboard bushing sealing lip faces outward from the bushing.
This allows excess grease to exit from the inboard side of the bushing and prevent it fro being pumped into the foundation assembly.

Question 84

Why should S-cam profiles never be lubricated?

Answer 84

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

Question 85

What should be the procedure for lubrication if the slack adjuster passes inspection?

Answer 85

Apply a thin film of chassis grease to the slack adjuster splines.
After reassembly, pressure-lubricate the slack adjuster according to the manufacturer’s instructions.
Pressure-lubricate the in-board S-cam bushing until grease flows out of the slack adjuster end of the tube.
Grease should not flow out of the out-board end of the tube.
If it does the seal is defective and it must be replaced.
Do not replace the existing grease fitting(s) with the pressure relief type.
Only standard non-vented fittings are to be used with spring-loaded lip seals.

Question 86

When checking hydraulic brake fluid in a vehicle what should be checked?

Answer 86

When checking hydraulic brake fluid, not only should the level be checked, but any leaks from the master cylinder reservoir, or brake line fittings as well.

Question 87

Why is the appearance of the fluid an important part of the inspection?

Answer 87

A cloudy or opaque look to the fluid will indicate some sort of contamination (dirt rust, etc.), especially if severe. any fluid that is cloudy or darkened should be flushed and refilled with fresh fluid.

Question 88

What would you look for when visually checking around the wheel cylinders?

Answer 88

Check for an accumulation of contamination, which generally indicates leakage.

Question 89

What is a thorough inspection of brake lines considered an important part of?

Answer 89

The hydraulic brake system.

Question 90

When checking the parking brake controls, what should you do?

Answer 90

Test actuator Freeplay and the parking brakes’ ability to effectively stop the vehicle.

Question 91

How should the brake pedal feel when checking its operation?

Answer 91

The brake pedal should feel firm with minimal free travel at the top of the stroke.
If it feels spongy it is usually an indication of air in the hydraulic system.

Question 92

What are trucks with hydraulic brakes manufactured after 1976 equipped with?

Answer 92

A tandem master cylinder, and dual circuit brakes.
In a dual circuit brake system, when one circuit fails, the remaining circuit will be unaffected.
When there is a failure in one circuit, the brake pedal will have excessive free travel.
A dash warning light should also be illuminated.

Question 93

What may be occurring if the pedal slowly drifts to the floor?

Answer 93

There may be an internal leak in the master cylinder or an external fluid leak.

Question 94

When the pedal is depressed, what should happen?

Answer 94

The brakes should apply promptly without excessive effort.

Question 95

What should be checked if excessive pedal effort is required to stop the truck?

Answer 95

The power brake booster.

Question 96

How should all of the wheels respond when the brakes are applied?

Answer 96

All of the wheels should brake evenly and the truck should not pull to either side.
If there is a pull, or one wheel locks up, or skids, the problem should be located and repaired before the truck is dispatched.

Question 97

When checking the wheel cylinders for an accumulation of contamination, which generally indicates leakage, what should you look for?

Answer 97

Look for evidence of leaks on the inboard side of the tire and wheel.

Question 98

When the drums are removed as part of a PMI, how should you check the wheel cylinders?

Answer 98

Peel back the rubber boot and check for fluid. Leaking calipers may be examined with the wheel removed.

Question 99

What can leakage from wheel cylinders result in?

Answer 99

Brake failure. Leaking wheel cylinders must be reconditioned or replaced.

Question 100

Most trucks are equipped with power brakes to provide shorter stopping distances with less effort. What provides power brake assist?

Answer 100

A vacuum booster, a pneumatic booster, or a hydraulic booster. Hydraulic boosters are more effective and more common.
When road testing the truck, it should stop without excess pedal effort.
There should also be reserve capacity, providing for at least one power-assisted brake application after the engine is off.

Question 101

What does the hydro-max power brake booster accomplish?

Answer 101

It engages an electro-hydraulic pump when there is an interruption of fluid flow from the engine-driven hydraulic pump.
The hydraulic power source may e the power steering pump or may be a dedicated hydraulic pump.

Question 102

Why do trucks with diesel engines and vacuum boosters need an engine-driven vacuum pump?

Answer 102

Because a diesel engine has pressure, not vacuum in the intake manifold.

Question 103

What does air over-hydraulic systems use for power assist?

Answer 103

Air pressure and the air in the tanks provide reserve capacity.

Question 104

What are the control valves required for braking balance with disc/drum combinations?

Answer 104

The metering valve delays the application of front disc brakes until the rear drum brakes are ready to apply.
If only the front brakes lock up or skid on slick surfaces when the brakes are lightly applied, the metering valve should be checked.

Question 105

How can you check the metering valve?

Answer 105

By installing pressure gauges on the front and rear brake circuits.
There should be no pressure at the front until the pressure at the rear reaches about 30-50 psi.

Question 106

Why is a proportioning valve needed?

Answer 106

To reduce rear-wheel lock-up during moderate to heavy brake applications.

Question 107

What do load-sensing proportioning valves do?

Answer 107

Allow more pressure to the rear wheels as the frame is lowered by heavier loads. The rear wheel should not lock up before the front.
There should be balanced braking at each of the truck’s wheels.

Question 108

What does a pressure differential valve do?

Answer 108

Turns on a dash warning light when a loss of pressure occurs in one half of a split hydraulic brake system.

Question 109

What do combination valves do?

Answer 109

Combine the metering, proportioning, and pressure differential valves into one assembly, which is replaced as a unit.

Question 110

How do you adjust hydraulic drum brakes?

Answer 110

1. Remove the backing plugs for measuring the shoe-to-drum clearance. Be sure that, if used, the parking brake chambers are caged.
2. Apply and release the brake pedal several times yo center the brake shoes.
3. Insert a feeler gauge through the backing plate opening and measure the clearance between the brake shoe and the drum. Typically this clearance should be 0.040 to 0.100 inch (1.01 to 2.5 mm)
4. If the shoe clearance is not correct, turn the manual adjusting heel (star wheel) on the wheel cylinder to obtain the correct clearance.

Question 111

Why should trucks equipped with hydraulic-assist power brakes (usually hydraulic pressure is supplied from the power steering pump) require back-up to supplement power braking?

Answer 111

It would be needed In the event the engine stops while the truck is moving.
This is accomplished by a flow switch, which senses the loss of hydraulic pressure from the power steering pump and actuates a 12-volt electro-hydraulic pump to provide emergency power assist.

Question 112

How can you test the hydraulic assist back-up system and warning devices?

Answer 112

On some systems, this is tested by making several brake applications with the key on and the engine off
When you have made enough pedal applications to exhau=st the pressure in the accumulator, you should get a warning light and busser, and hear the 12-volt electro-hydraulic pump engage.

Other systems are controlled by a solid-state alarm/brake booster module and can verify when they are operating normally by observing both a primary and a secondary brake light and a tone alarm.