Before driving it, suck/absorb out as much fluid as possible, by whatever means necessary, and top off with brake fluid. You don't want any of that stuff being pushed through your system. Ideally the mechanic would do that, but how often do things work ideally? Have it towed to the dealer, or drive incredibly slowly, with your foot ready to jam on the parking brake. If you lose the master cylinder, it's going to be hard to stop that thing.