Option 1: A complete '94-'95 Mustang non-ABS brake master cylinder system, including a brake block, came with the SN-95 rear disc brake conversion from MPS Auto Salvage. This could be swapped in and some brake lines would need to be modified, but nothing too difficult.
Option 2: Richard from North Race Cars advises that this is the easiest solution and works great. First unscrew the black plug on the front of the brake block, then remove the plunger and spring. Lastly replace the plug with a combination valve plug PN FMS M-2450-A. This part can also be purchased at your local auto parts store.