From your description, it looks like the O-ring seal, on the brake booster near the master brake cylinder, is bad. There is a square weep hole on the underside of the booster near the mounting bolts for the master cylinder. The O-ring can be replaced once the booster is removed from the car. There's an article by the Hack Mechanic of the Roundel, in the BMW Companion, that describes the removal of the booster. He doesn't, however, go into replacing the O-ring. The O-ring replacement is rather easy compared to the booster removal and the O-ring isn't difficult to match at your local industrial seal shop. If I remember correctly, Mformation had an article on the O-ring.