Why is my master cylinder needing to be replaced a second time on a "new" car? on 2012 Nissan Versa

Bought car in August 2012 and in January (after 2 months on order) they replaced the master cylinder. Told again today that it will need to be replaced again. Has anyone else had this issue? I don't understand why this is happening. It's under warranty but that's not the point, it's a relatively new car with repeat problems - brake pedal goes all the way to floor - a new car should not have this kind of trouble.

Asked by for the 2012 Nissan Versa