1. With the engine stopped, eliminate vacuum in the booster by pumping the brake pedal several times.
2. Push the pedal down and hold in this position.
3. Start the engine.
4. The booster is OK if the pedal drops further because of extra force produced.

If the brake pedal does not drop, the vacuum system, vacuum hoses, check valve, etc., is probably faulty and should be checked.

If no fault is revealed by checking the vacuum system, the fault is in the booster itself.