New Brake System Losing Pressure

[ChevyBrad]

I installed a new 1.032" Bore Strange engineering dual master cylinder, rear proportioning valve and new lines over to the right side frame from the master cylinder. I bled the master cylinder, tried bleeding the brakes by pumping and cracking the bleeders. Removed all the air I could and if you pump the pedal 3 times it holds real hard. But if you let it sit for a couple seconds it goes right to the floor. I was told I needed residual pressure valves and that should fix it. Installed and bled and same deal. My linkage travels from fully extended to bottoming out in the master cylinder so that's not the problem. I have since installed a new rear axle in the mean time and have the shoes adjusted up perfect. I switched out my master cylinder for my dads which is the same thing but with a 1.125" bore and still nothing. I borrowed a blue point vacuum bleeder from a Co-worker and now just barely have a pedal but not all the time... I am almost out of ideas... the original master cylinder worked great and I had not changed anything when I first installed the new master cylinder. Sorry for the long write up but, I needed all the detail. Any suggestions? Thanks.