Hi all. Just converted my 71 to front discs made by mbm. I put on all new stainless lines by inline tube and bled all the brakes. When the car is off I have a good stiff pedal. When I run the car the brake pedal is very soft and returns slowly. What can it be?

 

Most likely the booster or the master. If both of those are new from MBM just call them and talk through it.

Did you convert from manual to power?

 

Air still in the system is my guess

 

Are the bleeder screws on the calipers pointing up?