KbBlown01

I've got a frustrating problem with my brakes. When the car is stopped and the engine idling, I can push the brake pedal almost to the floor. It gives some resistance, then gives up and drops with a grinding feeling.

However, if I'm on a dirt road and lock up the wheels to the point of the ABS activating, the brake pedal is very hard. I think what this means is the problem causing the soft pedal is somewhere in the lines after the hydraulic control unit (hcu). Let me explain. When the hcu is decreasing line pressure to the locking wheel, a valve closes off the outlet port to that wheel. This makes the pedal only feel pressure from the master cylinder to the hcu, which would firm up the pedal.

I don't know how to troubleshoot this further as my knowledge of the hydroboost/abs braking system is limited (the chilton manual is no help here). Ideally I would like to plug the lines at various points for both the front and rear and perform the test. If the pedal is firm I could eliminate line sections up to the plug.

I have bled the brakes and master cylinder several times so unless there is air trapped somewhere in the hcu (which can only be done by a dealer), I've ruled out air in the system. Any help by a brake expert would be appreciated.