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Q: Brake pedals not firm halfway down

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My car's brakes have been fine until now. Recently, however, the brakes seem to "fade", or not have as much pressure when the brake pedal is halfway down. It will then firm up again. This happens when I'm driving. If the car is shut off, the pedal pump has plenty of pressure and stays firmly up. I guess that means the hydraulic part of it is fine.

I thought maybe the vacuum assist was the problem, since the problem only happens when vacuum is applied. I replaced the power assist, but the problem remains. The only other thing I can think of would be the master cylinder, but the pedal is fine when the engine is off. Why are my brakes or brake pedal less firm while driving?

Lack of brake power assist would typically result in a hard brake pedal, not one that sinks to the floor. My guess is that the master cylinder is faulty. A bad master cylinder will often result in a pedal that sinks to the floor.

Whatever the cause, this is a potentially dangerous situation that should be inspected firsthand by a professional. I suggest you contact the team at YourMechanic to inspect your uneven brake pressure since we make house calls.

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