Brake problem

[lphfh]

lphfh: having an issue with brakes on my AAR Cuda, I bought the car in parts so I used what I had. Has power disk/drum set up. I am just finishing up restoration. I have thoroughly bench bled the master cylinder and bled all the lines. When I push on pedal with engine off, I have a semi firm pedal but is goes to the floor slowly. When I have engine running (power assist) it goes to floor immediately. I have no leaks and have tried 3 different master cylinders because I thought I had one that wasn't holding pressure. I doubt if all three could be bad. Does anyone have an idea what could be going one. Possibly caliper issue or incorrect master cylinder push rod but I have been unable to find out what the correct length of the push rod is for power brake application

[xtopfuel]

the only way to detect where the issue is is to clamp the two front brake hoses and the rear hose with proper brake hose clamps (not vise grips) have someone hold pressure on the brake pedal and remove clamps one at a time, when the pedal drop or bypass occurs that's where the issue is, are you certain the rears are adjusted? are the calipers installed correctly? Bleeder at top? I have seen many install calipers upside down and impossible to get air out