I don't remember reading whether you'd checked for residual pressure valves in your front brake circuit?
If you search residual pressure valve, you'll find all the info. But basically, if the master was built for front drums, it may contain one that you need to remove. Worth checking for, at least. It can cause brakes to stick, but typically they'll stick, drag and stay stuck the entire time.. My car had them, and when I went to front disc I didn't check. Wouldn't roll in neutral on a modest hill.. Removed it and everything worked so much better.