So, I replaced the master cylinder. I started bleeding the system. When I bleed the front two brakes and the load sensing proportioning and by-pass valve, I have no problems. But BOTH of the back two brakes aren't producing any pressure when my helper presses on the brake pedal. Each stroke from the brake pedal is producing about 1/2" of fluid visible in the vinyl tubing I have attached to the bleed nipple. And it comes out S L O W . . .

When all the bleed screws are closed, the brake pedal goes to the floor with about equivalent effort as the clutch pedal.

Is it possible that I have so much air in the lines that I have a big air bubble that is compressing each time my helper presses the pedal? If this is the case, would a power bleed help?

Any other ideas?