I live an exciting life. The master failed on my 1983 300SD. I was able to get her stopped by some mad pumping as I approached a busy intersection. It went to the floor with no warning. The problem is that the master was rebuilt last year! So I ordered a new one last week and it happened again. There is no leakage from any line, caliper or master seals . Everything is dry. When bleeding the new master on the bench I noticed the far port that connects to the distribution block for the rear wheels didnt pump fluid, it just came out up the tube to the fluid level. The other two ports that go to the front wheels pumped up fine. Could this new master be bad also? Should there be a O-ring around the back going into the booster? Mine didnt have one on it when I removed it. Maybe the booster failed...not that it has anything to do with the master itself but as an assisting device. The brakes worked today when I moved the car by coasting it in.