I'm having the same problem, I have replaced both the booster and the master cylinder, and I still have the brakes(all 4) locking up. I have also tried to find a new push rod that goes between the booster and master. The GM dealers around me have no idea and have been very little help. Does anyone have any other ideas? I am about ready to cut the push rod down but would like to be able to find another one first in case it does not work....