I have problems with my 91 Chevy cavalier's brakes. When the motor is off, I have good brake pedal; but when I start the engine, the pedal goes to the floor. I have recently replaced shoes, calipers, lines, and yesterday t he second master cylinder. Same problem. There is a noise coming from the brake pedal. Just guessing, but checking to see if I should replace the b ooster. It could be bypassing the booster. Any thoughts?