I changed from power brakes to manual. My new master cylinder is from wilwood. I bench bled, installed, bled brakes and I cruised down my road, braking and when I turned around all 4 disc brakes locked up. I bleed a caliper and it rolls fine. The pressure is released until i do it all over again. I have tried 2 master cyclinders? The pedal is coming all the way back and rod length should be fine.
Are they wilwood calipers? If so they have to be centered up perfectly(or real close) since they do not float. You don't by chance have residual valves installed in there do you?
Sounds to me like the m/c pushrod is too long and not allowing fluid to return. Make sure that there is a little end play in the pushrod when fully released.......
Just a thought, maybe the original master cylinder had a vented cap on the master cylinder, and the wilwood master is one without a vented cap.When you apply the brake the pressure keeps building up in the master cylinder, keeping the brakes applied. I would call Wilwood and explain the situation and they will tell you exactly what is wrong, they deall with it every day. There number is 805-388-1188, hope this helps.