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1995 Civic DX. Brake pedal goes way down while driving and it takes longer to stop the car than is safe. New MC, brake booster is good, rear shoes are good and adjusted. No fluid leaks. System as been bled numerous times and fluid is clear with no air bubbles. The pedal is fine while parked and engine off. I've had the car jacked up completely off the ground with the pedal depressed and the wheels are firmly stopped. When I start it up it works fine, but once I go down the road a bit, the pedal goes nearly to the floor. Pumping the brakes helps somewhat. I replaced the pads in the front and took off the calipers and cleaned and greased the slide pins. The runout on the rotors is just above spec, but again, they hold well when the car is on jack stands. I have thoroughly checked the entire brake lines for leaks with none found and no fluid loss. The proportional valve has been replaced. I get 20 psi when checking the vacuum to the booster. I have never had such a problem and have dealt with brakes with a moderate amount of experience. I can't find any information on line that I haven't tried in order to fix the problem. We have only had the car for a short period of time so I don't have much of a history on it. Problem arose shortly after purchase. I am stumped as to the problem. I am not one to take it to a shop, as I fear they will throw parts at it and end up without a fix and/or cost too much. Any ideas are greatly appreciated!!

 
Rep Power: 209

Is the engine/vacuum hose to the master cylinder providing proper suction?

When you bled the brakes did you push the pedal to the floor repeatedly or stop at around 3/4 travel?
Depressing the pedal too far (past piston travel) can cause the piston to get stuck: https://www.knowyourparts.com/techni...-installation/

 
Rep Power: 0

I did check the vacuum line and it was working okay. I am going to do some more testing on it to make sure it is a steady vacuum.
Good point on the pedal. My grandson did the pedal depressing and I think it was pushed all the way down. Having said that, if the piston was stuck I wouldn't think I would have gotten a strong flow of fluid when bleeding the brakes. I did get a good flow on every brake. I will follow the instructions on the link you gave me and inspect the piston to be sure. Thanks for your input. It's good info to know now or for the future. I will update this thread in the next few days as I have been given other priorities for the next few days.