Your car uses 3 sources of vacuum. 1 is the engine, 2 is the reservoir, and 3 is the vacuum assist pump. You will have to see which of this is faulty, if any.
You could also have a broken vacuum line/hose or a bad vacuum element in the dash.
You will need some diagnostic work to find this. Consider a good independent that KNOWS MERCEDES.!.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS