need some help, In a bind here

My 67 barracuda has 74 Dart front disk and an aftermarket rear disk brake setup. All new, new master cylinder new proportioning valve new harn and soft lines. Her eis the problem and this is what has been done... So what would be next?

I can bleed the brakes and get a good pedal. Walk away for a few minutes and come back and the pedal will go to the floor.

I have swap out the master cylinder with a working one from another car same issue

I have blocked the back brake line off so only the front has fluid and the problem goes away. So I am thinking something in the rear. Nothing is leaking anywhere that I can see.

The rear system is the one that uses the 85 Cadillac calipers. It is like the fluid is returning back to the master cylinder and not holding the piston in the caliber in place. The proportioning vale is a disk to drum new one for a 74 dart.

What is next? Could it be in the valve? They are not cheap and getting it in and out with this big motor in it is not going to be easy.