I'm no brake expert, but could this have something to do with a residual pressure valve? The valves are sometimes inside the master cylinder and sometimes external. The normal pressure valve is 2 psi for disks and 10 psi for drums. I'm not sure if the wrong one would cause this condition or not. Probably not, now that I think about it. 10 psi doesn't amount to much. Just thinking out loud and trying to consider other causes. The posts above make more sense and would be the first thing I would check.
