I found the problem. Very strange. It probably explains why I had to pump the brakes with my old system. New system. Dual MC, front disc conversion, 2 psi residual valve for disc and 10 psi residual valve for rear shoes. I completed bleeding the system and I pumped up the brakes and they felt good. I came back a minute later and had to pump them up again and verified the calibers were still holding tight. My brake pedal has always been about 2 inches higher than the clutch pedal. Just by chance, I realized when pumping the brake, if I only allowed the pedal to return to about the same height as the clutch pedal, the system held pressure. I installed a block under the floor that allowed to pedal to only return to the same height as the clutch pedal. This solved the problem of needing to pump the brakes up and the locking calipers. I am guessing, but I think the MC plunger was coming too far back internally allowing the rear line to drain back into the master cylinder and blocking the port for the front line causing it to hold pressure on the calibers. Google says this is not possible but can happen in reverse. The internal plunger staying too far forward. I have no real explanation but they work now.
