It depends on what your problem is.
IF you are having problems with the engine not shutting down quickly, it could be problems with the shut off diaphram on the rear of the pump or the vacuum switch that is on the ignition switch.
You will have to test either one with a vacuum pump/gauge.
PS, the valve is located ON the igition switch so you will have to remove it to get to the valve.
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MERCEDES SPECIALIST 11 YRS