Brakes

I have a 1994 Honda Civic EX, 4 door automatic with approx 130k miles and ABS brakes. I have an intermittent brake problem where the brake pedal sometimes goes to the floor. I can drive in traffic and the brakes will hold for a few stops and then and at another stop, the brakes will hold for a few seconds and then drift to the floor. They will hold again after pumping a couple of times, and then the process repeats. I've bled all of the air out of the system, 1 wheel at a time. Is the master cylinder bad, or could air somehow be getting into the system in the course of stop-and-go traffic? Would this have anything to do with the "low pressure side" or the "high pressure side"?