bad master cyl??

[nasty72nova]

I have completed the installation of my 4 wheel disk brake kit on my nova. I'm using a 7/8 bore master cyl from wilwood. I have zero leaks. Double and triple checked every fitting. I have bleed the brakes roughly ten full cycles using the old fashion pump,pump,pump hold method and a pressure bleeder. I get zero air out of the bleeders. It was bench bleed and has no air in the master cyl. If I pump it up I get a halfway decent pedal. As soon as I move the car 50 ft or more the pedal goes to the floor. Sound like a bad MC?????? Remember I see zero leaks. All new lines and brake parts. Nothing is old.