Testing both the pump and the SOV are DIY jobs with a hand vacuum tester. With the engine running, hand pump into the main vacuum line from the vacuum pump and measure the vacuum shown on the gauge. It should be >18", preferably >20"

With the engine still running, disconnect the vacuum line from the SOV, connect your hand pump and pump the handle. It should stop the engine and the valve should hold vacuum.

$20 and 5 minutes of your time for a mightyvac diagnosis, or $200 diagnostic fees + labor for a mechanic to do it. Your money, your time... If you intend to keep the car for a long time, that little $20 hand pump will get used again, and again, and again.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]