Very strange brake dilemma!

[67chevy_hotrod]

Getting sort of desperate boys, I need my truck to be back to 100%.. no back brake pressure at all and very soft pedal. The fronts will work but only when my foot is buried in the pedal. All new brake lines, everything is new. When bleeding you get loads of brake fluid. It's all been bleed, multiple times there is no air in it. What I am understanding is I have the volume but not the pressure.

My truck is a 1969 3/4 ton Longhorn. I put performance online front disc brakes on it. It's got a 1972 GMC Dana 60 rear drum. In THEORY(I realize aftermarket parts aren't stock), all this is essentially stock 1971/1972 equipment. Now I've gone to Napa and picked up a 1969 3/4ton booster and master cylinder and installed. For a proportioning valve, I used a part identical to the one POL sent me, the new one is from RPC. My first question is will the pre '71 ( I say this year cause it's first year of disc brakes) booster/mc setup work with my current brake setup? Is the input rod the same length? My second question is what reservoir goes to what brakes on the 3/4 ton MC? I understand it's opposite of a 1/2 ton? Is that difference in front to back in the MC or proportioning valve? Currently I have front reservoir going to front on prop . valve and rear to rear. Is that backwards? Any help is appreciated, the truck is not safe to drive right now and I need it. Thanks again!