Vacuum Booster Diagnosis

WARNING: This page is about a different car, the 2004 Suzuki Swift + and 2004 Chevrolet Aveo. However, it is still accessible from the selected car via links, so may be relevant.

With the engine stopped, eliminate the vacuum in the booster by pumping the brake pedal several times.
Push the pedal down and hold it in this position.
Start the engine.
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 defective and should be checked.
If no defect is revealed by checking the vacuum system, the defect is in the booster itself.