Vacuum Brake Booster: Testing and Inspection

Brake Booster Test

Functional Test

1. With the ignition switch in LOCK (0), press the brake pedal several times to deplete the vacuum reservoir, then press the brake pedal hard and hold it for 15 seconds. If the brake pedal sinks, either the master cylinder is bypassing internally or the brake system is leaking. Inspect the brake hoses and lines Testing and Inspection.

2. Start the engine with the brake pedal pressed. If the brake pedal sinks slightly, the vacuum booster is operating normally. If the brake pedal height does not vary, do the brake system test Component Tests and General Diagnostics.

3. With the engine running, press the brake pedal lightly and shift the transmission to D. Apply just enough pressure to hold back automatic transmission creep. If the brake pedal sinks more than 10 mm (0.39 in) in 3 minutes, the master cylinder is faulty. A slight change in brake pedal height when the A/C compressor cycles on and off is normal (The A/C compressor load changes the vacuum available to the booster.).

Leak Test

1. Press the brake pedal with the engine running, then turn the ignition switch to LOCK (0). The brake pedal height should not vary while pressed for 30 seconds.

- If the pedal height rises, go to step 6.
- If it does not rise, go to step 2.

2. Start the engine and let it idle for 30 seconds. Turn the ignition switch to LOCK (0), and wait 30 seconds. Press the brake pedal several times using normal pressure. When the pedal is first pressed, it should be low. On consecutive applications, the pedal height should gradually rise.

- If it rises, the booster is OK.
- If it does not rise, go to step 3.

3. Disconnect the brake booster vacuum hose (A) at the booster. The check valve is built into the hose.

NOTE: If the check valve is faulty, replace the brake booster vacuum hose/check valve as an assembly.






4. Start the engine, and let it idle. There should be vacuum available.

- If no vacuum is available, the check valve is not working properly. Replace the brake booster vacuum hose and the check valve, and retest.
- If vacuum is found, go to step 5.

5. With the ignition switch at LOCK (0), reconnect the vacuum hose to the brake booster.

6. Start the engine, and then pinch the brake booster vacuum hose between the check valve and the booster.

7. Turn the ignition switch to LOCK (0), and wait 30 seconds. Press the brake pedal several times using normal pressure. When the pedal is first pressed, it should be low. On consecutive applications, the pedal height should gradually rise.

- If the pedal position does not vary, inspect the O-ring between the master cylinder and the booster. If the O-ring is OK, replace the brake booster.
- If the pedal position varies, replace the brake booster vacuum hose/check valve as an assembly.