I've had that problem - I solved it by boring the fluid port in the master slightly larger. the problem seems to have been caused by the replacement cup having a slightly longer skirt than the original and it wasn't retracting far enough to uncover the forward port (from reservoir into the cylinder), so fluid in the master cylinder could not return to the reservoir when the piston was retracted as far as it could go. Corrosion in the wheel cylinder can cause the same brake drag symptom, that the piston doesn't retract - but releasing some fluid wouldn't get the piston to retract. (boy, can they get crusty - change the fluid every year or so, and take the wheel cylinder off and turn it upside down to do it. The crap - and water settles to the bottom). If you want some details on wheel cylinder rebuild, I have a file somewhere - ask about it.