Hi all, I’ve had some strange brake issues on a MCS with ~120k. The pedal is incredibly stiff while the car is off. I recently did pads, rotors and a master cylinder. When the car was running the pedal would slowly sink to the floor. I put my motive on it and put 10lbs on it and found some light leakage. Assuming master cylinder, I replaced it. Obviously, bench bled and did a full bleed twice. PThe previous symptoms continue to occur. The brake pumps up very well when the car is off. Then when running, the brake sinks and won’t hold pressure. My next guess is that this is potentially a brake check valve but I’m not too familiar with the car as I just purchased it.
Any help of troubleshooting recommendations would be greatly appreciated.

 

I was told that is normal with ABS cars. A slight bleed through the ABS module.

 

I would agree with that, if it was only slight. I should be more clear that I lose probably 60-70% of the pedal, but I am able to "pump it up" while driving and coming to stop, then it drops significantly lower and lower. It honestly almost feels like when you overheat your brakes on track and they fade out from under you. One bit of reading indicated the potential for a failed Diaphragm inside of the booster, or a faulty vacuum pump? I just want to make sure I'm looking in the right places and not replacing an entire brake system to find out it's a simple valve somewhere.

 

Have you made sure you're not pushing fluid out somewhere? Failing that, my next thought was a failing booster or vacuum pump. The vacuum pump and line should be easy enough to check. The line uses quick connect fitting with seals inside. Maybe one of the seals is starting to give up?

 

Just so you know the vacuum pump on my R56 ceased and took out 4 valves.

I was not the owner then , but might be good to pull it and check it out. Make sure its getting oil and not metal to metal.

 

Don't believe it's related to the booster or vacuum pump - The pedal pushes a rod that pushes the master cylinder piston. The force of this push action is amplified by the booster using the vacuum of produced by the vacuum pump. A failing booster or vacuum pump should make the necessary force increase, but not lead to a sinking pedal.

Especially since you removed the master you will most probably got air into your ABS unit. Did you run the ISTA procedure to cycle your ABS pump?

 

I missed the part about replacing the master cylinder...