About a week ago my old focus ZTW wagon was creeping up when stopped at traffic lights. Master cylinder I thought. It took me a couple days to get to the mechanic and in the meantime it didn't do it anymore, I had good brake pedal. Well, I had the master cylinder replaced anyway. When I picked up the car, the mechanic said he had trouble bleeding the system: the right rear wheel kept giving him trouble with getting good pedal back. Anyway he said he thought it was OK and I drove home the 25 miles. On the way I got stuck in traffic and the brake pedal starting feeling mushy. After a couple more stops it went to the floor and I barely had any brakes. I used the hand brake for about 5 minutes and the pedal came back to normal. I thought maybe the system "burped": it had air in one of the lines and now it was gone. The car did fine for a couple more days. Well, today it did it again, I lost my brakes in slow traffic, and then they came back. What is going on? I don't think the master cylinder was the problem. I obviously need to get back to the mechanic but its tough with my work schedule, and I'd like to know what you think about the cause.