One problem related to brake fluid leak has been reported for the 2011 BMW Z4. The most recently reported issues are listed below.
When exiting a parking lot into a city street, when the brakes where pressed, a loud pop noise was heard, then a hissing noise and the brake pedal sank to the floor. There was no braking power after that and my car rolled into the street out of control, it was a scary situation but no accident or collision ocurred at this time. To stop the car the brake pedal had to be pressed very hard all the way to the floor,there was no leakeage of brake fluid. The BMW dealership diagnosed a faulty brake booster.