I figured something else was wrong with the system, so I've already swapped out the master cylinder and proportioning valve. It had a parts store remanufactured master cylinder on it with an unknown bore size, so I went with one of the new wilwood mopar 4 bolt compact master cylinders with their recommended 15/16" bore size. I also just installed a new Dana 60 that came with the 12.19" wilwood rear disc kit, so that will rule out any possibility with the original rear drum brakes that were on it.