Grand Cherokee 2WD V8-4.7L VIN J (2004)
Brake Master Cylinder: Testing and Inspection
DIAGNOSIS AND TESTING - MASTER CYLINDER/POWER BOOSTER
NOTE: Inspect and repair any external fluid leaks before performing test.
1. Start engine and check booster vacuum hose connections. A hissing noise indicates vacuum leak. Correct any vacuum leak before proceeding.
2. Stop engine and shift transmission into Neutral.
3. Pump brake pedal until all vacuum reserve in booster is depleted.
4. Press and hold brake pedal under light foot pressure. The pedal should hold firm, if the pedal falls away the master cylinder or HCU may be faulty
(internal leakage).
5. Start engine and note pedal action. It should fall away slightly under light foot pressure then hold firm. If no pedal action is discernible, power
booster, vacuum supply, or vacuum check valve is faulty. Proceed to the POWER BOOSTER VACUUM TEST.
6. If the POWER BOOSTER VACUUM TEST passes, rebuild booster vacuum reserve as follows: Release brake pedal. Increase engine speed to
1500 rpm, close the throttle and turn off the engine.
7. Wait a minimum of 90 seconds and try brake action again. Booster should provide two or more vacuum assisted pedal applications. If vacuum
assist is not provided, some component of the booster is faulty.
POWER BOOSTER VACUUM TEST
Booster Vacuum Test Connections
1. Connect vacuum gauge to booster check valve with short length of hose and T-fitting.
2. Start and run engine at curb idle speed for one minute.
3. Observe the vacuum supply. If vacuum supply is not adequate, repair vacuum supply.
4. Clamp hose shut between vacuum source and check valve.
5. Stop engine and observe vacuum gauge.
6. If vacuum drops more than one inch HO (33 millibars) within 15 seconds, booster diaphragm, check valve or check valve seal/grommet is faulty.
POWER BOOSTER CHECK VALVE TEST
1. Disconnect vacuum hose from check valve.
2. Remove check valve and valve seal from booster.
3. Use a hand operated vacuum pump for test.