The brakes in my '91 accord have a problem that couldn't be fixed by bleeding the lines. It's something that happens every so many days, or weeks. At times the break pedal goes all the way to the floor. It usually happens when traveling at a low rate of speed, 40 mph and under. Pumping the brakes will usually bring them back to normal, but not always. Usually it doesn't happen after the car has been parked and restarted. This has been going on for the past 6 months without any regular pattern of how often. I don't see any fluid leaks, and the brake fluid level is fine. A co-worker of mine recommended that I check the equalizing valve. But by reading what the equalizing valve is for, I don't think that the problem is caused by it. Does anybody have an idea of what may be wrong with the brakes? Should I replace the master cylinder? Thanks!