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.

[red and rusted]

That prop valve does not work like you think it does. When you have an issue like a broken brake line or just bleeding them, the piston in the prop valve will shift illuminating the brake light on the dash. that is where the wire goes.

Also gravity bleeding is the only way to go.

[l79m20]

I tend to disagree with how the proportioning valve works. Vehicles with front disk brakes and rear drums have proportioning valves to dictate how much pressure is sent to where. If you sent the same amount of pressure to all 4 corners you wouldnt get brake everywhere, just to the front. The valve allows the rears to catch first then the fronts to bite.

[red and rusted]

That is a better way to describe it.

[l79m20]

swapped out the master cylinder and still have the same problem. Took apart a booster that I had tried on the truck and it looks great inside. I dont understand what the problem is. Can anyone help. I dont want to spend $115 on a booster and then have the same problem. I am fairly certain the booster is fine.

[MTCK]

I'm sure you are probably all over it but did you bench bleed the master cylinders? I've bought a couple trucks where the PO sold it with a new MC and couldn't figure out what was wrong with the brakes and seeming inability to get a hard pedal and a good bench bleed fixed them right up. Is your brake light on? If not does it work?

[l79m20]

yeah, got that all done, got all of the air out before I ever installed it. The dash brake light is on. Blocked off all the vacuum ports and i can get brake.