Rally Z

Hi, I have a 1990 RS with a stock rear-end with drum brakes. I just pulled it and installed a Borg Warner 9 bolt with disc brakes. I bled the system and tried using the stock proportional valve but the pedal goes to the floor. So I replaced it with a stock proportional valve out of an 1986 Firebird. Here's my problem, The Firebird valve has fine threads and my car has coarse threads. Anyone experience this and how did you fix it? Or do I need to go with a master cylinder for rear disc brakes? Thanks in advance!