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Fading Brake

1988 full size bronco. Randonly the brakes fade to the floor. The vehicle stops but the pedal is on the floor. In the past two months have replaced the master cylinder twice, replaced the brake booster and the ABS value. Problem remains. Can yoiu help? Thank you

Did you bench bleed the MC?

When you pump the pedal, does it get harder?

May want to take a close look at all the vacuum hoses.

I’m inclined to discount the vacuum booster as a possibility for this one. Its diaphragm assists in pushing the center rod that moves the master pistons in the master cylinder, but it cannot allow the travel of the pedal to randomly go all the way down. The only thing that can do that is air in the system, which is why I suggested bench bleeding of the MC. If air is entrapped in the MA chambers it could move around and randomly get compressed. It being smaller than the fluid, it might even be able to get forced past the piston seals.

Plug the master cylinder outlets and stand on the pedal for a minute. If the pedal stays high and hard the master cylinder is OK. If it fades it is bad. New does not = good.