I have a ‘97 4.2 standard cab which has develop a very confusing brake problem. Recently my brakes started pumping up at higher speed. My brake pedal becomes so stiff that pressure is being applied to the brakes without me even touching the pedal. When running slow such as simple neighborhood driving everything is fine. The brake pedal returns to a normal feel and of course no drag on the actual pads themselves. I tried everything from adjusting the brake push rod which adjust the stiffness of the brake pedal (I like about a 1” play on the pedal before it starts to apply the brakes) to replacing the rotors and pads to replacing the master cylinder. I also replaced the booster pump along with a new check valve. I’ve also bled the brakes many times too. It's as if at higher RPM the check valve is allowing/putting too much pressure on the booster which in turn is forcing the brake rod into the master cylinder which is then applying the brakes and of course the brake pedal in the cab reflects this and it’s stiff as ****. There's no play at all in the pedal when it happens.
Now anyone that's ever adjusted the brake rod knows that if you adjust if too far out your pedal gets stiff and the brake pads will drag on the rotors without even toughing the pedal. But this is happening only at higher speed.

Totally confused. Any help would be greatly appreciated.

 

My theory is that the ABS pump assembly is what's building up the pressure. How it's doing it I can't really say, but it is a pump with the ability to pressurize the brake lines.

 

Wanted to say thanks for the reply. At first I didn't really know how to test it but after watching a lot of youtube videos on all kinds of ABS sytems one guy suggested just unplugging the ABS pump. Of course you immediately get the ABS light but it bypasses the ABS system without having to due anything else. I took it out on the interstate and at about 75 no problem at all. That ABS pump is pretty expensive so I'm headed over to the local junk yard this WE.

Thanks again.

 

Good, at least you can drive the vehicle without burning up your pads and rotors.