76 brake problems

[l79m20]

For my build I took a long bed half ton and turned it into a short bed 3/4 ton. Ran all new brake lines from each corner back to proportioning valve. Had an issue bleeding the brakes, saw some fluid leaking from rear drums so I changed out the rear cylinders. Bled again and got a great stiff pedal. As soon as truck is started the brakes go to the floor. Took off the booster and swapped for a known good setup. Bled again, same issue. The proportioning valve has a power wire run to it. This is what makes me think it is the issue, because I have power until the truck has power. I did read some articles that said when bleeding the brakes you need to have the key on, and the "button" on the prop valve pushed in so it will even out the valve. We tried that and it didnt fix the problem. I have also heard that you can pull that brown wire off the prop valve and that will allow the brakes to work but without being "power". I did not get to give that a whirl yet. Any ideas or solutions would be helpful.

I have bled the brakes several time and am confident all the air is out of the lines.