Checking the vacuum booster unit

7. To check the efficiency of the amplifier, press the brake pedal four or five times, then start the engine. When starting the engine, the pedal should noticeably «succumb» due to the increase in vacuum. Let the engine run for at least two minutes and then turn it off. If you now press the brake pedal again, a hiss should be heard from the amplifier. After about four to five strokes, the hissing should stop and the pedal will be pressed much harder.