Along with his post I have a 93 buick lesabre with the same problem. i have been able to get a master cylinder for 52 bucks from auto zone. Or you can buy a new one from the dealership for around 312, although i think autozone is even selling the new ones at 90 bucks.
My question is how do you go about bleeding the breaks during this replacement process. and can you do it by yourself in a day