bad brake booster

while doing restoration work on a 1968 plymouth gtx,i purchased a master cylinder and booster assembly from jims auto parts.
bad mistake. the booster will not release vacuum and because i contacted jims auto parts after 30 days of purchase,they were glad to be off the hook and not even interested in how one of their rebuilt boosters was not working.
i recommend purchasing mopar parts anywhere but jims auto parts.
so im asking what am i missing or do i need to replace the rebuilt with swomeone else. thanks bobby