Mechanical Parking Brake problems of the 2002 Mazda Tribute

The brakes failed on 11/22/02 at 16,714 miles. The dealership changed the master cylinder as defected. On 1/03/03 at 19,632 miles the brakes failed again. Again the dealer changed the master cylinder as defective. I questioned whether this was the cause since it happened twice and didn't feel this was the root cause. On 1/10/03 at 20,634 miles the brakes failed again. In all three cases the brake pedal goes to the floor and will not stop the vehicle. The daelership now says its an abs problem. The emergency brake does not stop the vehicle either.