I have a 2007 versa hatch, while driving the other day the brake pedal went to the floor. i have pressure when the car is off but when its on it goes to the metal. No recent brake work has been done so we thought maybe the fluid was low. The fluid is full. i have an appointment with meineke tomorrow , but want to make sure i know what to look for. I was told it could be the master brake cylinder or maybe air in the line. does anyone have any ideas where to look ? it just happened so suddenly that it confuses me .