Brake Master Cylinder problems of the 2008 Mazda Tribute

At various times between August 2015 and November 2015, my brakes did not respond properly. At about 3-week intervals, my brake pedal would go farther down toward the floorboard than it usually takes for me to stop. This usually happened at low speeds (5mph to 40mph), usually when I am approaching a stoplight or stop sign. After all of these instances, whenever I parked my car and started a new trip, the brakes would go back to normal. However, on 11/10/15, the brake pedal went all the way to the floor when I tried to stop at 15-20mph. This was on a busy city street, and I had to drive up on the median to avoid hitting the car in front of me. I took it to sears and was told there was no problem with the brakes; however, they bled the brake line, changed the brake fluid. Then, I took it to the Mazda dealer and was told there was no problem with the brakes, but that the fluid needed to be changed. When I took it to carmax, they told me it was the master cylinder, and they replaced it along with the front abs tone rings. On 11/25/15, two days after getting my car back from the carmax mechanics, the brake pedal was again feeling like it was going farther down than necessary, and then it went to the floor again. That time, I was approaching a stop sign in a residential neighborhood, so it was at a low speed. The car is now back at carmax, and they have called to say that they cannot find any problem. The scariest things about this (besides the obvious fear that my car will cause an accident) are: 1) the random nature of these brake problems; 2) the fact that the brake pedal goes back to normal after one of these events; 3) mechanics were not spotting a problem with my brakes; and 4) when a problem was specified and supposedly fixed, the brakes still went out.