Soft brake pedal with engine running

mike_dunn · 04-20-2009, 03:29 PM

With the engine off I can pump up and get a hard pedal, can't reach the floor with it. Engine on the pedal drops and becomes wimpy with lots of travel and no back pressure and pumping it does nothing. I can even slam the pedal to the floor while cruising and not lock the brakes. I've bled and bled and bled with super blue (using speed bleeders and old fashioned method) til I see no micro bubbles, the pads are seated firmly on the rotors, and there aren't any leaks. Is it trapped air or maybe a faulty master cylinder? Before I drop any $ anything else I should check?

pozee · 04-20-2009, 03:33 PM

What was your bleeding sequence? Did you bleed farthest fitting out from the MC to closest?

mike_dunn · 04-20-2009, 03:57 PM

Yea followed Bentley manual, RR-LR-RF-LF. Used a mallet on the calipers too.

terryb · 06-24-2009, 06:36 PM

Did you figure out the problem? Was it the M/C?

docrodg · 06-25-2009, 02:46 AM

I am assuming you did not let it run out of fluid at any time. That would require a bench bleed of the MC.

Pump up the pedal with the car off, then with pedal firm start the car, pedal should drop a little and stay firm (this is the power assist working right). If pedal does not move you have a bad power assist. If power assist ok then it sounds like bad cup seal in the master cylinder and a rebuild or replace is needed.

4G1 · 06-25-2009, 06:38 AM

could it be a vaccum issue?

docrodg · 06-25-2009, 07:12 AM

Quote:

Originally Posted by 4G1

could it be a vaccum issue?

That is what the boost check will find out. If there is no vacuum then there will be no sinking of the pedal. Then you just troubleshoot the boost system (hose connected and with vacuum, fittings etc. ok) If vacuum is getting into booster then the diaphragm is blown and the booster needs replaced.