I'm a newcomer here, and a new owner of a 41 Chevy 3/4 ton pickup that I plan to fully restore (my first restoration). One of my first tasks is to rebuild the brakes. I've installed new brake hoses, shoes, wheel cylinders, and master cylinder. I bench bled the M.C. before installing. I've bled the brakes until no more air is coming out. I see no leaks in the system anywhere. The shoes/wheel cylinders have been properly adjusted. I can pump the brake pedal several times and the brakes are solid as long as I hold pressure with my foot. When I let off on the pressure for a few seconds and then press the pedal again, it goes to the floor. Why is the system not holding pressure? Any help/advice would be appreciated.

My toys:
'41 Chevy Model AL 3/4 Ton
'40 Chevy Model KD 3/4 Ton
Hinomoto 209N Tractor/Backhoe
Norwood LM2000 Sawmill
PJ 16' Trailer
and a Brand New Shop!
Be what you is, not what you ain't, 'cause when you is what you ain't, then you ain't what you is.