Cherokee 2WD L4-150 2.5L VIN P MFI (1991)
Brake Master Cylinder: Testing and Inspection
1. Start engine and check booster vacuum hose connections. 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. If pedal holds firm, proceed to step 5. If pedal does not hold firm and falls away, master
cylinder is faulty (internal leakage)
5. Start engine and note pedal action. If pedal falls away slightly under light foot pressure then holds firm, proceed to step 6. If no pedal action is
discernible, power booster or vacuum check valve is faulty. Install known good check valve and repeat steps 2 through 5.
6. Rebuild booster vacuum reserve as follows: Release brake pedal. Increase engine speed to 1500 rpm, close the throttle and immediately turn off
ignition.
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, perform booster and check valve vacuum tests.