Sounds like you have run one of the segments of the reservoir for brake fluid on top of the master cylinder dry. Are any of your warning lights coming on? Should be one for low fluid and one for ABS malfunction. If not you may have an electrical problem too.

I believe all MB brakes in the last fifty years have a twin circuit so if a line breaks or leaks the whole braking function is not lost. The compromise is that one back brake line is fed from one compartment in the brake fluid reservoir and the other one from the other compartment. Check the fluid reservoir carefully. It has a vertical partition behind the fill opening intended to prevent the leaking circuit from draining all the fluid. It comes up pretty close to the top of the reservoir, which means you have to nearly fill the reservoir up into the filler cap neck, especially if the front of the car is sloped "down" compared to the rear.

Hope this helps. Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)