We have a '91 911. We just changed the brake fluid, something that we have done several times before with no problem. However, this time, when bleeding the air from the breakline, the right rear brake, (where we started) wouldn't pump-up fully. It did pump up but didn't seem right. After bleeding all four we found that the brake pedal goes to the floor the first time we press it. They seem "mushy". This has never happened before. We went back and "re-bled" the lines and the front one are strong but the two back aren't "right". Could this be the master cylinder?
Chances are the master cylinder has failed. If you were bleeding the brakes by pumping the pedal, often times, esp with older cars the master cylinder seals begin to leak internally. Replacing the master cylinder will probably solve your issue. Hope this helps.