RESIDUAL VALVE IN MC?
Mine were just doing this with my Wilwood stuff...after I blead them good it seemed to help. If that MC was not made for disc brakes it will have a residual valve in it that keeps preasure on the drum brakes to keep the springs from forcing fluid back to MC...you probably already know that.. :? just a thought in case you hadn't thought about that...that or MC is junk.