Power Brake Unit
WARNING: This page is about a different car, the 1993 Mercedes-Benz 190E. However, it is still accessible from the selected car via links, so may be relevant.
NOTE:
Before servicing power brake unit, inspect check valve. Determine whether problem is caused by power brake unit or check valve.
Manufacturer does not recommend disassembly. If problem is found in power brake unit, replace complete assembly.