When will it stop? Brake question

[K2Orion]

I have a 72 C10 custom deluxe that I bought this summer that I can't stop. I'm running out of ideas and I want to make sure I'm not missing something before I throw any more parts at it. So here's some history.

The truck was originally a 72 C10 custom deluxe with a 350/350 2wd with tilt and air. I bought a 4wd truck with a 350/350/203 amd 44/60 with 3.73s.

When I bought the truck I knew it had a few issues. The brakes would not stop the truck when it was in gear and would barely stop it when it was in neutral. I assumed it needed a master cylinder.

First, I tried bleeding the system. After 2 qts. of dot3 the system ran clear and had no bubbles. No change in pedal feel or stopping ability.
FAIL

Second, I found some hard lines that were rusted and started leaking after some movement. I replaced them and bled the system again.
FAIL

Third, I discovered that the truck was missing a proportioning valve. I bought a used MC and PV from a board member (whose name I forgot) that came from a 72 C10. I installed these and bled the system again.
FAIL

Fourth, I bought a reman. MC for a 72 K20 but it would not fit. It used a different length pushrod and reversed the position of the lines from the MC to the PV. So I bought a MC for a C10 and installed it. I bled the system again.
FAIL

The first 2 attempts I just bled the system by having somebody depress the pedal as I cracked the bleeder screw. Each time the fluid ran clear with no bubbles.

The second 2 attempts I vacuum bled the system with a Mity-vac.
I am going back to the garage to try to put together a pressure bleed setup. I hope that will work.

So what should I try next? Replace the fex lines? New calipers? The previous owner daily drove the truck as I bought it for a while before he parked it. Then it sat for 10 years. But this combo of parts should work, right? I shouldn't need a MC for a K20, should I?

Help?