Brake help!

[aziznowarinty]

69 c10, i bought it last week. The truck has been converted to front discs. The front calipers are locked up and have been since i bought it. I changed the master cylinder i bought one for a 71 . And after much doing i got all the wheels to bleed. It took standing on the pedal to make them bleed. Bleeding freed the calipers. But you push the pedal once and the calipers lock back up. Could the combination valve be sticking? A don't suspect brake hose because both sides lock up. Any ideas? How hard is it to reset a combination valve? How is it done? Any help is greatly appreciated. I would like to drive the thing soon.