Front Calibers Dragging

The front brake calibers are locking up on a hydro boost braking system. Initially, I had no brakes. Pulled the master cylinder top off and the gasket was defective so I put a new one on and the brakes came up without having to bled the system. The back drum brakes are not affected when this happens. All the blogs that I have read indicates if the proportion valve was defective, the back brakes would be sticking.

I changed the calibers and still this mystery continues to happen. When the calibers stick, the brake pedal is hard/stiff like a board to the point that you can not apply the brakes yourself. The couple of times this happened, I used the park brake and believe it or not, it seem to correct the problem.

Called the manufacture of there hydro boost system and they seem to think that the inline filter is dirty. But I am confused that the front brake only is sticking.

Any comments will be greatly appreciated