Booster Functional Test; Accumulator Leakdown Test - Hummer H1 Service Manual

7-8 Brake System

Booster Functional Test

With the engine off, apply the brake pedal several times until
the accumulator is completely depleted. Depress the brake
pedal using 40 lb-ft (54 N•m) 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. Replace the hydro-
booster.
3. If the accumulator can be heard charging and discharging
but does not hold a charge, replace the hydro-booster.
4. Deplete the accumulator by pressing the brake pedal
several times. If the accumulator can has lost its charge, it
is possible to rotate or wobble the accumulator can with
respect to the housing. Replace the hydro-booster.
Handling - The booster should not be carried by the accumula-
tor nor should the booster ever be dropped on the accumulator.
The snap ring which holds the accumulator into the housing
should be checked for proper positioning before the booster is
used. The accumulator contains high pressure gas and with any
high pressure gas a certain degree of danger is present if mis-
handled.
Disposal - The accumulator should not be exposed to exces-
sive heat, fire or incineration. Before discarding accumulator
following replacement, drill a 1/16 inch diameter hole in the
end of accumulator can to relieve the pressure. BE SAFE! Pro-
tect your eyes. Wear approved safety glasses.
SERVICE BRAKE SYSTEM BLEEDING
NOTE: If only the front or rear half of the system has been ser-
viced, it is usually necessary to bleed only that half of the sys-
tem. However, if a firm brake pedal cannot be obtained after
bleeding, it will be necessary to bleed the entire system. The
brake hydraulic system can be bled manually or by using a
pressure tank and adapters. Each method is outlined in the fol-
lowing procedures.
WARNING: Disconnect the batteries before bleeding
the brake system. If the ABS modulator receives power
during the bleed procedure, air may become trapped in
areas in the modulator that cannot be bled by conven-
tional techniques. The modulator would need to be re-
placed to obtain a firm brake pedal.
3-1-01
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WARNING: If the reservoir has run very low or dry,
bleed the master cylinder before bleeding the remainder
of the brake system to keep from introducing additional
air into the system.
WARNING: Always wear eye protection when bleeding
brakes. Failure to do this may cause injury if brake fluid
comes in contact with eyes.
Pressure Bleeding
CAUTION: When using a pressure bleeding tank, (J–29532)
follow the manufacturer's instructions for its use. Use only
DOT 3 brake fluids when bleeding. Do not exceed the recom-
mended working pressure when pressurizing the tank. A tank
pressure of 15-20 psi (103-138 kPa) is sufficient to bleed the
brake hydraulic system. Release all air pressure from the tank
after using it.
NOTE: This procedure covers bleeding at one wheel. Repeat
bleeding task for remaining wheels.
1. Disconnect batteries.
2. Remove cap from master cylinder. Fill master cylinder if
necessary.
3. Install pressure tank bleeder adapter to master cylinder
(Figure 7-4).
4. Connect line from pressure tank to adapter.
NOTE: Bleed calipers in the following order: right rear, left
rear, right front, left front.
5. Remove protective cap from bleeder screw on caliper
assembly (Figure 7-5).
6. Connect short piece of hose to bleeder screw, and place
other end of hose in container 3/4 full of brake fluid.
7. Open valve on line from pressure tank to master cylinder
allowing pressurized brake fluid to enter system
(Figure 7-4).
8. Open bleeder screw 3/4 turn and observe brake fluid in
container. Close bleeder screw when brake fluid flows free
of air bubbles (Figure 7-5).
9. Disconnect hose from bleeder screw and install protective
cap on bleeder screw.
10. Close valve on line from pressure tank to master cylinder
(Figure 7-4).
11. Disconnect line from adapter.
12. Remove adapter from master cylinder and fill master
cylinder if necessary.
13. Install master cylinder cap.
14. Connect batteries.
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