Each fuel pump has a critical check valve mounted at the pump outlet. The purpose of this check valve is to prevent fuel from leaking back through the pump after the engine has been shut down. Constant fuel pressure must be held in the fuel system for the engine to restart quickly. If one or both check valves are not seating properly then your engine may take a lot of cranking to start. This is particularly common problem after the engine has sat overnight.
Picture replacing fuel pump check valve.

Fits the following models. Note only 124 chassis 124.030 - from A 368937 and on. Does NOT fit 190E 16 Valve!!!!