1969 Chevy Nova Strange Brake Problem

I replaced the wheel cylinders and shoes and after atempting to bleed the barkes for several hours and could not I changed the master cylinder even though I did not think it had gone bad at the same time I changed the wheel components. I bench bled the master cylinder and all was fine. I put it on the car and began to bleed the rear brakes and all seemed fine. When I started bleeding the front brakes I continued for several hours to get a mix of fluid and air. Finally I stopped and now I can pump the pedal and get a full, firm pedal and as long as I hold pressur on the pedal it holds. When I release the pedal and depress it again it goes completly to the floor. Also I should add that in the middle of all this I changed the combination valve and Still have the same problem. Any Advise?