My brake pedal turns soft after I start the car

I assume that you have already tried at least the two-man bleeding process before reading this article. If the brake pedal on your car turns soft (depresses) after you turn on the engine, try the following:

1) Recall symptom history -- when did the problem appear?

2) Inspect that brake hardware was installed correctly. Inspect that all brake guide pins are smooth, and can be moved by your finger. The brake booster turns on when the engine starts. Even if the pedal is hard with the engine off, any small problems with brakes will mean that the higher force applied due to the brake booster will cause a larger pedal traveled, compared to when the engine is off. Bent, non-uniformly worn, and rust-frozen brake hardware may show these symptoms.

2) When you have installed brake hardware, did you install it correctly? Are calipers right side up? Are drum brake bleed nipples located above the brake line? Incorrect installation may place the bleed nipple below the brake line, and air bubbles will collect there.

3) There may be trapped air in the ABS system. If only handyman's methods of bleeding are available, then activate the ABS three times by driving over unpaved or graveled road, and by applying brakes swiftly. DO NOT do this on asphalt, grass, wet road, or with a car behind you. Rebleed.

4) If you have only done the two-man bleeding method, you will need to try professional bleeding. However, I have found that many independent garages are not yet equipped with ABS scanners which can command the ABS pump to cycle. These scanners are usually make-specific. You might have to go to the dealer. If you have a German car (Audi, VW), then you can use VAG yourself to cycle ABS. Note: independent garages will promise you that they can properly bleed the ABS system, but never trust them on their word. Many can't.


Remember: If you can pump up the brakes (by depressing the brake pedal several times) and the pedal gets firmer, then you still have air in the system. However, if you apply constant pressure to a depressed brake pedal, and it slowly drops, then you likely have a MC (master cylinder) bad o-ring.

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Page last modified 25-Apr-12 21:53:51 EDT
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