1969 C10 Disc Brake Conversion, Only Front Brakes will Work???

[matthew20061]

While restoring my 69' C10 which originally had manual 4 drum brakes, I converted the front brakes to disc brakes, new calipers, rotors, pads, and hoses. I bought a new master cyl from Car Quest for a 72 Chevy with factory front disc brakes. Then I got a proportioning valve from CPP. Before installing the master cyl I bench bleed it very good so that no bubbles were coming out. After plumbing new lines from the new master cyl to the proportioning valve, I bleed all the brakes starting with the passenger rear, drivers rear, moved my way up to the front passenger side and driver side. So after pressure bleeding them, I only had front brakes. So I gravity bled them and slide the prop valve back to center and tried them again. Same thing instant only front brakes. Its been this way for the last year. I've gone thu like a gallon of brake fluid or more just in bleeding these things so I'm almost 110% sure theres no air in the lines. Brake pedal is also hard, but when I slide the prop valve back to center and jamb something in there to keep it centered they feel great just like normal. But it always slides back over and blocks off the rear brakes???

I have access to an original working master cyl and original factory proportioning valve from a 72 Cheyenne should I swap this on there and see if it fixes it?

So what could cause this? Bad master cyl? Bad proportioning valve?? Or?