Two things come to mind, air bubbles WILL form if you're running DOT 5 brake fluid or some synthetics (its called cavitation). ABS MUST use DOT 3 or 4 (per OEM) to avoid this. But when 3050 mentioned the leak.......I had an issue. My slave cylinder for the clutch had a pin hole leak, and since the slave and master cylinder on my 325is share a common reservoir, guess what (yup, low fluid). And it was the tinniest of leaks too, barely detectable as it would drip out the weep hole under the bell housing while driving. If yours is manual I'd certainly look there. Other option have the brake fluid changed. Most shops have a power bleeder that will suck all the old fluid out the bottom while dumping new stuff in the top and bleeding the system at the same time......the last little kicker, most OEM's actually recommend that service every couple of years. Of course if its due you might as well get the brake pads replaced at the same time.