Disc brake swap, now no brakes? Pedal to the floor when truck is turned on.

[72freak]

We have done brakes on 100's of vehicles here at Rottenwood Garage so I am fairly certain I have the procedure down. LOL. HOWEVER, we are having trouble with a 69 C10 2x4 that we just converted from power drums to front disc. The kit he ordered came from Brothers with all the brake components, spindles and the prop valve. We used the booster I had put on the truck when I converted it to power drums a few years ago and it works fine. We bought a 72 disc brake master cylinder to complete the system. Its all together, no leaks, bled out with no air in the lines and the rear brakes are weak and when you turn the truck on so the booster is working the pedal goes to the floor. We bleed it out while not running and end up with a solid pedal but as soon as you start the truck it goes to the floor and at that point there is no rear brakes and the front seem to grab a little. We did bench bleed the master and all of the flexible hoses and rear brake components are newer..We are stumped. We are going to put a fresh set of eyes on it this morning when all the guys come over so we may get it figured out but I wanted to see if any of you have had the same experience and what was your solution..