C900 brake booster diagnosis (vacuum servo assist)
There are three tests to accurately diagnose the condition of your brake booster.
Booster function (vacuum assist) - with engine off - depress brake pedal once or twice firmly and hold, start engine. Pedal should sink towards floor 2-3 inches. If it doesn't booster is faulty.
Air tightness check - with engine off - pump brake pedal several times. With each subsequent pump pedal travel should decrease. If it doesn't brake booster is faulty.
Air tightness under load - with engine on - depress brake pedal once firmly and hold. Turn engine off whilst still holding brake pedal. Pedal should not rise or sink for 45 seconds. If it does brake booster is faulty.
If your booster fails any of these 3 tests it is faulty and needs to be replaced.
Replacing the booster takes about 2 hours and is fairly easy. Remanufactured boosters range from $120-$180.