Vacuum Brake Booster: Testing and Inspection

POWER BOOSTER FUNCTIONAL CHECK
1. With the engine off, eliminate vacuum in the booster by pumping the brake pedal several times.
2. Push the pedal down and hold it in this position.
3. Start the engine.
4. If the pedal drops further, because of the extra force produced, the booster is OK.
If the brake pedal does not drop, the vacuum system (vacuum hoses, check valve, etc.) is probably defective and should be checked.
If no defect is revealed by checking the vacuum system, the defect is in the booster itself.