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Brake System and Diagnosis

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Irish_Alley:
COMBINATION VALVE
The combination valve is comprised of three sections, each serving a different function (figure 7). The metering or hold off section of the valve limits the pressure to the front disc brakes until a predetermined front input pressure is reached, enough to overcome the rear shoe retractor springs. There is no restriction to the inlet pressures below 20 kPa (3 psi) to allow for pressure equalization during the no apply periods.

The proportioning section of the combination valve proportions outlet pressure to the rear brakes after a predetermined rear input pressure has been reached. This is done to prevent rear wheel lock-up on the vehicles with light rear wheel loads. The valve is designed to have a By-Pass feature
which assures full system pressure to the rear brakes in the event of a front brake system malfunction, also full front pressure is retained in the event of rear malfunction. The pressure differential warning switch is designed to constantly compare front and rear brake pressure from the master cylinder and energize the warning lamp on the dash in the event of a front or rear system malfunction. The valve and switch are so designed that the switch will latch in the “ warning” position once a malfunction has occurred. The only way the lamp can be turned off is to repair the malfunction and apply a pedal force as required to develop about 3102 kPa (450 psi) line pressure.



http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/X8632_1986_GMC_Light_Duty_Truck_CK_G_P_10_to_30_Service_Manual.pdf#page=432

Irish_Alley:
ROAD TESTING THE BRAKES
The brakes should be tested on a dry, clean, reasonably smooth and level roadway. A true test of brake performance cannot be made if the roadway is wet, greasy, or covered with loose dirt so that all the tires do not grip the road equally. The testing will be adversely affected if the roadway is crowned so as to throw the weight of the vehicle toward the wheels on one side or if the roadway is so rough that the wheels tend to bounce.

Test the brakes at different vehicle speeds with both light and heavy pressure; however, avoid locking the wheels and sliding the tires on the roadway. Locked wheels and sliding tires do not indicate brake efficiency since heavily braked turning wheels will stop the vehicle in less distance than locked wheels. More tire-to-road friction is present with a heavily braked turning tire than with a sliding tire.

EXTERNAL CONDITIONS THAT AFFECT
BRAKE PERFORMANCE

1. Tires—Tires having unequal contact and grip on the road will cause unequal braking. The tires must be equally inflated and the tread pattern of the right and left tires must be about equal.
2. Vehicle L o a d in g —When the vehicle has unequal loading, the most heavily loaded wheels require more braking power than the others.
3. Front Wheel Bearings—Loose front wheel bearings permit the disc to tilt and have spotty contact with the linings causing erratic action.
4. Front End Alignment—Misalignment of the front end, particularly in regard to limits on camber and caster, will cause the brakes to pull to one side.

HYDRO-BOOST SYSTEM TESTS
The Hydro-Boost system receives its source of power from the power steering system. Therefore a faulty power steering system may affect the operation of the booster, just as a problem in the booster may affect the steering system. Prior to performing any tests the following checks must be made.
1. Check all the power steering and brake pipe connections for leaks or restriction.
NOTICE: Power steering fluid and brake fluid
cannot be mixed. If brake seals contact steering fluid or steering seals contact brake fluid, seal damage will result.
2. Check and fill the brake master cylinder with brake fluid.
3. Check and fill the power steering reservior with power steering fluid. If fluid contains air refer to POWER STEERING (Sec. 3B3) for further diagnosis.
4. Check power steering pump belt for wear and tension. Adjust if needed, refer to POWER STEERING (Sec. 3B3).
5. Check engine idle speed, refer to the vehicles Emission Control Information label for idle specifications.
6. Check steering pump pressure. Refer to POWER
STEERING (Sec. 3B3).

NOISE DIAGNOSIS
The following noises are associated with the
Hydro-Boost and may or may not be cause for
customer complaint. Some noises are normal and for
the most part temporary in nature. Other noises may
be a sign of excessive wear or the presence of air in
either the booster or the steering system.
1. A moan or low frequency hum usually accompanied by a vibration in the pedal or steering column may be observed during parking maneuvers or other low speed maneuvers. This may be caused by a low fluid level in the power steering pump or by air in the fluid. Holding the pump at relief pressure (steering wheel held all the way in one direction) for more than five seconds will cause air to enter the system. Check the fluid level and fill if needed. The system must then sit for one hour to remove the air. If the condition persists, refer to POWER STEERING (Sec. 3B3).
2. A high speed fluid noise may be heard when the brake pedal is fully depressed, this condition is normal.
3. Whenever the accumulator pressure is used, a slight hiss may be noticed. It is the sound of the hydraulic fluid escaping through the accumulator valve, and is completely normal.
4. After the accumulator has been emptied, and the engine is started again, another hissing sound may be heard during the first brake application or the first steering maneuver. This is caused by the fluid rushing through the accumulator charging orifice. It is normal and will only be heard once after the accumulator is emptied. However, if this sound continues, even though no apparent accumulator pressure assist was made, it could be an indication that the accumulator is not holding pressure and should be checked using the procedure for the “ Accumulator Leakdown Test” in this section.

BOOSTER FUNCTIONAL TEST
With the engine off, apply the brake pedal several times until the accumulator is completely depleted. Depress the brake pedal using 180 N (40 lbs.) of force and start the engine. The pedal will fall and then push back against your foot.

ACCUMULATOR LEAKDOWN TEST
1. Start the engine and charge the accumulator by applying the brake pedal or by turning the steering wheel from stop to stop. Turn off the engine and let the vehicle sit for one hour. After one hour there should be at least two power assisted applications with the engine off.
2. If the reserve system will not retain a charge for one hour, but functions normally immediately following charging, the accumulator valves are at fault and the booster must be disassembled and the accumulator valves replaced.
3. If the accumulator can be heard charging and discharging, but it does not hold a charge, disassemble the booster and replace the accumulator valves.
4. Deplete the accumulator by pressing the brake pedal several times. If the accumulator can has lost its gas charge, it is possible to rotate or wobble the accumulator can with respect to the housing. Replace the accumulator assembly.

SEAL LEAK DIAGNOSIS (FIGURE 1)
A. INPUT ROD SEAL. A damaged seal will show up as a fluid leak from the mounting bracket vent hole. The booster must be removed from the vehicle and disassembled. The input rod bore should be checked for any scratches that may cause the leak. If scratches are present, the
housing cover must be replaced. If no excessive scratches are present, then the booster seal kit can be used to replace the appropriate seals.
B. POWER PISTON SEAL. Power piston seal damage will be noticed by fluid leaking out at the common master cylinder brake booster vent and possible reduction in power assist. The booster must be removed from the vehicle and disassembled. The piston should be checked for any scratches that may be the cause of the leak. If scratches are present, then the booster seal kit can be used to replace the appropriate seals.
C. HOUSING SEAL. If the housing seal is damaged, fluid will leak out from between the two housings. The booster must be removed from the vehicle and disassembled. The booster seal kit should be used to replace the housing and input rod and power piston seals.
D. SPOOL VALVE PLUG SEAL. Damage to this seal will be noticed by fluid leaking out past the plug. The booster need not be removed from the vehicle.
E. ACCUMULATOR SEAL. Damage to this seal will result in fluid leakage past the accumulator cap.
The seal can be replaced while the booster is installed on the vehicle.
F. RETURN PORT FITTING. Tighten the fitting to 10 N m (7 ft. lbs.). If the leak continues, replace the seal ring under the fitting.

http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/X8632_1986_GMC_Light_Duty_Truck_CK_G_P_10_to_30_Service_Manual.pdf#page=422

Irish_Alley:

Irish_Alley:

Irish_Alley:
BLEEDING THE
HYDRO-BOOST SYSTEM
Whenever the booster is removed and reinstalled, the steering system should be bled.
NOTICE: The power steering fluid and brake fluid cannot be mixed. If the brake seals
contact steering fluid or the steering seals contact brake fluid, seal damage will result.
1. Fill the power steering pump reservoir to the proper level, let the fluid remain undisturbed for
at least two minutes.
2. Start the engine and run momentarily.
• Add fluid, if necessary.
3. Repeat steps 1 and 2 until the fluid level remains constant after running the engine.
4. Raise the front of the vehicle so the wheels are off the ground.
• Support the vehicle with suitable safety stands.
5. Turn the wheels from stop to stop, lightly contacting the stops.
• Add fluid, if necessary.
6. Lower the vehicle.
7. Start the engine and depress the brake pedal serveral times while rotating the steering wheel from stop to stop.
8. Turn the engine off and then pump the brake pedal 4-5 times.
9. Check fluid level, add fluid if necessary.
10. If the fluid is extremely foamy, allow the vehicle to stand a few minutes with the engine off. Then repeat steps 7, 8 and 9.
11. Check for the presence of air in the oil. oil will have a milky appearance. Air in the system will also cause the fluid level in the pump to rise when the engine is turned off. If it becomes obvious that the pump will not bleed the air after a few attempts, refer to POWER STEERING (Sec.3B3) for futher diagnosis,

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