still having brake problems:(

I've been trying to get a good pedal on my brakes for about two weeks now. I've swapped three master cylinders, first one was a remanufactured unit from autozone. Second one was a 15/16 bore from doctor diff and the last one is the 1 1/8 from doctor diff. All three have the same symptoms which leads me to believe my issue is not the master cylinder. I can pump up the brakes and get a good pedal that holds but if I release it for 5 seconds it goes to the floor with just a little pressure building at the end of the stroke. I have bled the system several times with a power bleeder from harborfreight. It is the unit that creates vacuum with a compressor hooked up to it. Works very well and I have probably bled close to a gallon of brake fluid through the lines. I even tried to gravity bleed all four corners. I swapped out the rear drums a while back for disc and have not had a good pedal since. Also using a wilwood proportioning valve to the rear brakes that was there with the drums.No visible leaks and I am stumped. I am going to swap all the lines in the near future it's the only parts that are not new and I'm thinking it can't hurt to put new lines in to replace the 50 yr old ones:) Wilwood disc up front that have been there for about three yrs with no issues. I can't help but think I am missing something:( Any help would be greatly appreciated. Sorry for the long post just wanted to make sure I give enough info.