1985 W123 300TD
How do you test the changeover valve for proper operation? I am talking about the valve with three vacuum ports that is located behind the brake booster next to the fuse box; not the one near the oil filter. I am trying to trace a vacuum leak. Should it hold vacuum on any of the 3 ports with a mighty vac? It has an electrical connection so is it tested with the key on or off? Vacuum leaks are frustrating to trace for me!

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1985 300TDT
2013 E350 Bluetec