Here's an interesting problem. I have an 89 240 (230,000K, orig motor & trans) that I use as a back-up car. I don't drive it much. It has some quirks but it generally runs real nice. I hadn't driven it in about 6 months when I had to use it while my other car was being inspected. The brakes [expectedly] creaked and srcraped when I first set out, but worked fine. After a while, I noticed that distinct smell of overheated pad material, you know, the smell when a quarry truck has to stop quickly. And there WAS a truck in front of me, which I thought was the source. I came to a normal stop and turned on to a road (still behind the truck) and traveled for about 5 miles. Approaching a stop sign, I applied the brakes and the pedal went to the floor with only light resistance. The car wasn't breaking. The ensuing adreneline rush had me pumping the pedal and throwing it in low as I headed toward the stopped truck in front of me. Thank God it was a slight uphill incline. I managed to stop before needing to use the emergency brake. I did a U-turn and pulled into the first driveway (I happened to know the guy). I had absolutly no brakes at that point. Free pedal travel (no resistance) to the floor. No fluid leaking anywhere that i could see and the resevoir stayed full, even when pumping. The R/F and L/R wheel were very hot (remember, I hadn't braked for a good 5 miles).
Called AAA for a tow. While I waited for the truck, the homeowner and I tinkered around speculating on the cause which we figured was probably the master cylinder. Checking the pedal occasionally. after about 1/2 hour. I tried the pedal once more and the brakes suddenly were working fine again! I still let the garage tow me home anyway as there were a few dangerous hills (dangerous if you don't have brakes!). So, I'm still inclined to think it's the MC, but I'd like to hear any other suggestions or theories about how the brakes can utterly fail one minute and come back to be fine the next. I'm a fairly experienced mechanic for many years, and I have a few theories, but I'd like to hear other opinions, Thanks & sorry for the length! BTW There was a MOST bizarre coincidence as well if anyone feels like hearing it.