I have been fighting an issue with my disc/drum set up since I finished it. I replaced everything front to back. MC, front disc kit and proportioning valve for a 71 a body with front disc from Dr.Diff. Ran all new lines made of Cupro Nickel 3/16" line and added new brake hardware, hoses and shoes on the rear, the drums didn't have a ridge so I left them as is. The car will stop amazing but then the pedal feels as if it sucks air, then when it sits after use the pedal sinks to the floor once and then comes back up. I have bench bled the master multiple times and ran probably a gallon of fluid through the system. Along with going through every fitting to check for leaks, I added a 10lb residual pressure valve to the rear circuit and called Cass to see what his thoughts were, he believed it to be a bad MC and sent me another one. I bench bled it an then installed, re-bled the system once again and was having the same issues. It is always on the rear circuit, I never get air from the front. I'm about at my whits end. I'm unsure as to what I should do. Does anyone have any ideas? I have been fighting this issue for months and get to it whenever I can, I even took it to two different mechanics after I came to a roadblock and they said I hadn't bled it enough but the problem persisted afterward. I have zero fluid leaks from the brakes, no wet spots or stains from brake fluid. I hope one of you guys can give me a few pointers, because I'm at my wits end. The only thing I haven't done is change the distribution block, but I asked Cass and he said it wouldn't effect the system in such a way. Do I just have more bleeding to do or is something amiss? My next step was making a weekend of it and going through all the flares in the system and re-bleeding. But I would prefer to spend my time with the old lady if I could avoid it and cruise around the Coronet instead of wrenching non stop on the same issue. Thanks in advance for any help, I'm scratching my head on this one.