Brake Bleeding Anomaly on my 66

[Steveoreno]

I installed a power disc brake kit on a 66 C-10 that I am building, the entire brake system is new, front to back, every piece. I have bled the brakes three different ways, vacuum, gravity, and even had the Warden do the old "pump and hold" . In each case the results are the same, good solid pedal with about 1 1/2" of travel until I start the motor, at which time the pedal goes nearly to the floor. I have triple checked the entire system and there are no leaks. I do have the proportioning shuttle locked in place as I didn't want it to move during my bleeding attempts. I cannot explain it, never had this happen before. Any ideas as to what would cause this? Thanks. -Steveoreno-