Brake Issues - 85 K20

[Moloko]

So I bought this 85 K20 back in June that was left at a local shop due to it's brake lines rusting out. I replaced the master which was leaking, all the brake lines, rear wheel cylinders, and front calipers. After I bleed the brakes (both normal, and with a Phoenix reverse bleeder) It has pedal when the truck is off, and will pump up and hold pressure. When I turn the truck on the pedal goes right to the floor, and the prop valve trips to the front. I went as far as to re-change the rear brake line again tonight thinking it was leaking at the junctions, and it still trips the valve the second you touch the brake pedal.

Could the prop valve be bad? I've never heard of it, I've only heard of people changing them when they won't re-center after tripping, but I've been taking it off and manually pushing it back with a pick each time.