All drums and no hope.

[ChevyGearHeads]

I have a '68 C10 with an all manual drum set-up. I have tore down and replaced every piece of the system (including two master cylinders) and I get some pressure but not for long and the pedal works its way to the floor. The only existing original part is the proportioning/combination valve/distribution block. I have tried to bleed this truck about 4 times for hours on end with the same bleak result. Any guidance would be great.