need help diagnosing brake issue

[GallopinGhost]

Got a brake problem that has me stumped. I have a challenger stocker that I've built from the ground up. Everything in the brake system is new. The master cylinder and 4 wheel discs are from strange(I've replaced the MC once, no change). It has a hurst line loc, brake pressure switch, and a pressure gauge plumbed into the dash. No proportioning valve.
The problem is it seems to have air in the system. If the brakes sit for an hr or so, the pedal goes just about to the floor. Pump them up, feels great, an hr later, junk. I have pressure bleed them many times using quarts of fluid.

Here's the crazy part. To diagnose, I removed the brake lines from the MC and put in a loop. Pedal feels great. Removed the loop and plugged one port and re-connected one line. Pedal feels great. Put the plug in the other port and connected the other brake line, pedal feels great. Attached both lines, pedal is almost to the floor......