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 he engine.
4. The booster is OK if the pedal drops further because of the extra force produced.

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 found by checking the vacuum system, the defect is in the booster itself.