One problem related to brake master cylinder has been reported for the 2003 Mercedes Benz C320. The most recently reported issues are listed below.
Braking system failure my wife was driving to work. She attempted to stop at an intersection, her foot went to the floor while braking, red light and warn buzzer went off. . . No brakes. Luckily and thank god, she avoided being t-boned in the intersection and was able to get it into a parking lot and down shift to use e-brake. The car has only 70,000miles on it. The repair of the system sounds like it is $2500+. Not happy at all since mb knows it is a problem and should have a replacement braking system for it or repair. This is not just a old master cylinder going out. This is a whole sensor unit failing.