Brake Master Cylinder problems of the 2010 Toyota Matrix

Brake pedal suddenly felt soft as if there was air in the brake line. Brake only worked when you pumped fast. Otherwise the pedal would go down to the floor. I took the car to the dealership, they verified the complaint, and fixed the problem for free. They replaced the master cylinder and fluid. Brakes were fine until 3 weeks ago, April 23, 2012. Exactly the same happened, thank goodness as I was driving slowly. Took the car back in and they acknowledged the problem. They concluded it was a faulty part. They replaced the master cylinder again (covered by warranty) and the brakes were fine again. Then today, may 9, 2012, the brakes failed again, exactly the same problem, thank god no accident. My car is in the shop again so I don't know yet what their conclusion is. The scary thing here is that this is the third time the brakes failed within 6 months. Have you had complaints like this before? has there been a recall? I appreciate your consideration and look forward to your response. [xxx] information redacted pursuant to the freedom of information act (foia), 5 u. S. C. 552(b)(6).