Hydraulic Brakes Diagnosis and Repair Flashcards
Where does the potential energy of a hydraulic brake system come from?
The potential energy of a hydraulic brake system comes from the mechanical force created by the action of a driver’s foot on a brake pedal, usually assisted proportionally by pedal geometry leverage and a power-assist system.
What can be assumed in any hydraulic circuit?
The hydraulic medium is not compressible.
What happens if force is mechanically applied to a liquid in a closed system?
It will be transmitted equally by the liquid to all parts of the system.
What happens if forced is applied by a master cylinder?
It will transmitted equally throughout the hydraulic system.
Valves are used to modify that pressure in some locations.
What does the hydraulic circuit of a hydraulic brake system consist of?
A master cylinder, proportioning valves, metering valves, pressure differential valve, and wheel cylinders.
All truck hydraulic brake systems have dual circuits. What does this mean?
In the event of a failure in one of the circuits, the other will back it up.
As in truck air brake systems, the circuit are defined as the primary and secondary circuits.
What can cause poor stopping?
Poor s
What can cause noise?
Noise can be caused by loose or incorrectly installed components.
What can be the cause of pulling, grabbing, and dragging?
sticking valves and collapsed hoses.
What can cause hard brake pedal concerns?
Hard brake pedal concerns can be caused by a faulty booster.
What can cause soft brake pedal concerns?
can be caused by air in the system, a leak, or a bypassing master cylinder.
What would you use to test and determine the cause of pedal concerns?
Use a set of hydraulic gauges to determine the cause of the concern and the repair needed.
What may be used to test pressure values within the hydraulic circuit?
pressure gauges.
How can the hydraulic system circuit be pressurized?
The hydraulic system circuit can be pressurized simply by starting the vehicle engine and applying the brakes by foot pressure.
How can you verify external leaks?
External leaks may be verified by cleaning the externally visible portions of the circuit and applying the brakes.
How can you verify internal leaks?
Internal leaks are more difficult to locate.
Internal leakage within a master cylinder can be verified by using gauges plumbed to each portion of the hydraulic circuit.
What would you use to retrieve and diagnose fault codes?
A handheld scanner or a PC with the appropriate software.
The tool is connected to the ATA data link, which is usually located in the cab typically to the left of the steering column.
Fault codes can be diagnosed using the manufacturer’s troubleshooting trees.
What do most truck hydraulic brake system, use the brake pedal assembly to do?
Add leverage to the mechanical force provided by the driver’s foot pressure.
What happens if you have insufficient pushrod or a pushrod that is adjusted too tight?
Can force the master cylinder pistons forward slightly, blocking the compensating ports.
This prevent fluid flow between the master cylinder bore and the reservoir, which can cause brake pressure buildup from heat expansion of the fluid resulting in brake drag.
Freeplay should be adjusted to the manufacturer’s specification.
How are the primary and secondary circuits in the brake system actuated?
The master cylinder converts the mechanical force applied to it by driver foot pressure and the brake booster system into hydraulic pressure.
What do the primary and secondary circuits of the brake system consist of?
It usually consists of integral reservoirs (one for each circuit), cylinder housing, compensating ports, return springs, and primary and secondary pistons.
How is the primary piston actuated?
The primary piston is actuated mechanically.
Pressure developed in the primary portion of the master cylinder charges the primary circuit (this can actuate either the front or rear ) and the secondary piston.
When is the secondary piston actuated?
Is actuated hydraulically by whatever pressure value is developed in the primary portion of the cylinder to charge the secondary circuit.
What happens when the mechanical force applied to the primary piston is relieved?
Return springs acting on both the primary and secondary pistons return them to their original positions, permitting the fluid applied to each circuit to return to the reservoirs.
How are both sections of the master cylinder aspirated?
They are aspirated with brake fluid by fill and compensating ports.
How is each piston sealed in its bore?
Each piston is sealed in its bore by rubber seals.
Primary and secondary circuit fluid do not come into contact with each other under normal operation.
What happens when a failure occurs in either circuit?
The pressure differential switch will illuminate the red lamp in the dash the first time the brakes are applied following the failure.
What happens when a failure occurs in the primary circuit?
The primary piston will be forced through its travel without generating any fluid pressure until it contacts the secondary piston and mechanically actuates the secondary circuit
What happens if a failure occurs in the secondary piston
The primary circuit will function normally, but the actuation of the secondary piston will result in no pressure delivered to the secondary circuit.
What should you do to the vehicle in either case of a primary or secondary failure?
The vehicle should be brought to an immediate standstill and not operated until a repair has been performed.
What should you use when testing a master cylinder in a hydraulic braking system?
When testing a master cylinder in a hydraulic braking system, a liquid-filled hydraulic test gauge should be used.
What can cause sludge and particulate that can plug fill and compensating ports, resulting in slow application times, slow release time, and brake failure?
Deteriorated fluid, deteriorated seals, or a mixture of incompatible fluids.
Master cylinders are generally serviced by replacement for time and convenience reasons but they can also be ________________.
Reconditioned.
What will swell the seals and leave behind corrosive residues?
Use of any petroleum-based lubricants or solvents.
When replacing a master cylinder, what should be checked?
The brake pedal pushrod length should be checked when replacing a master cylinder.
What will a pushrod adjusted too long cause?
A pushrod adjusted can cause the brakes to drag.
What will a pushrod adjusted too short cause?
A pushrod adjusted too short can prevent sufficient travel of the master cylinder piston.
When inspecting and replacing brake lines, flexible hoses, and fittings, what should steel tubing be checked for?
Steel tubing should be checked for wear, dents, kinks, and corrosion.
What is the benefit of preflared and preformed tubing?
Preflared and preformed tubing helps reduce custom cuts, bending, and flaring of new tubing.
What are two types of flaring styles and seats?
International Organization for Standardization (ISO) and double flare.
ISO uses an outward flare, while a double flare creates a double wall at the nipple seat for greater strength.
What should you clean brake tubing with and why?
Only isopropyl alcohol should be used because of its ability to evaporate rapidly and residue-free drying.
Residues remain when using other cleaning agents such as soap and water or mineral spirits.
What should use to bend tubes?
A tube bender.
What will kinked lines cause?
Kinked lines create flow restrictions that can affect brake performance.
When should you replace flexible hose?
Flexible hose should be replaced when visibly cracked, bulges under pressure, or is leaking.
What should you do if brake fluid is contaminated with a petroleum-based liquid?
The flexible hoses and all other rubber components of the brake system must be replaced.
How should brake lines be routed?
Brake lines should be routed away from all moving components and care should be taken to ensure that nothing will contact the lines during suspension jounce and rebound or during steering.
why is a metering valve used on vehicles equipped with front disc and rear drum brakes?
It is required to achieve brake timing balance during light brake applications by withholding the delivery of application pressure to the front disc brakes until pressure exceeds a predetermined value in the circuit responsible for actuating the rear brakes.
Why is a lag or delayed required through withholding the delivery of application pressure to the front disc brakes?
The lag or delay is required so that hydraulic pressure builds sufficiently in the rear brake hydraulic circuit to overcome the tension of the rear brake shoe return springs and the free travel the shoes.
The objective is to enable simultaneous application of both front and rear brakes.
For this reason, the metering valve is sometimes known as a hold-off valve.
What should you do when using a pressure bleeder to bleed any system equipped with a metering valve?
The manufacturer’s instructions as to how to open the valve must be observed.
What should you do when manually bleeding a brake system?
When manually bleeding a brake system, application of the brake pedal develops sufficient pressure to overcome the metering valve opening pressure.
What is a portioning valve used on and what is its functioning?
A proportioning valve is used on systems combining front disc and rear drums.
the proportioning valve is installed in the circuit supplying the rear brakes.
Its function is to reduce the application pressure to the rear wheel cylinders and prevent rear wheel lockup.
When should the proportioning valve be verified?
The proportioning valve should be verified at each brake inspection or if reported for a rear wheel lockup condition.
To check valve operation, hydraulic gauges should be installed ahead and behind the valve.
Why is a load-or-height-sensing valve used on some systems?
To sense vehicle load transfer effect under braking.
How does a load or height sensing valve work?
The valve proportions front and rear braking, correlating it to weight transfer during braking..
Where is load or height sensing valve located?
The valve is located on the vehicle frame cross member and is activated through a linkage system connected to the rear axle housing.
How are the metering (hold-off) load sensing/proportioning, proportioning, and combination valves checked?
These valves are checked by installing pressure gauges at each wheel end and comparing gauge pressure when applying and releasing the brakes.
A faulty valve can prevent pressure buildup at one or more wheels, cause pressure to build too quickly, or trap the pressure and cause the brake to drag.
What does the pressure differential valve consist of?
The pressure differential valve is also known as a brake light warning valve.
It consists of a cylinder through which primary and secondary hydraulic pressure acts on either side of a spool.
What happens when the pressure in both the primary and secondary circuits is equal?
The spool floats in a neutral position.
What happens when a pressure imbalance occur in either the primary secondary circuit?
The spool will shuttle to one side of the cylinder, and in doing so grounds an electrical signal, illuminating a dash warning lamp.
