Others have described this better than I can and with photos, (do a search) but if you kill the ignition switch and suck on a tube hooked up to your shutoff valve (brown tube near the oil filter housing connected to a metal disc shape) and can't kill the engine - it's the valve. Likely you will kill it that way, so start moving up the line. Check the vacuum on the brown line, then plug all the other connections on the T, then move upstream towards the vacuum pump.

Good luck -

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Current Mercedes
1979 maple yellow 240D 4-speed

Gone and fondly remembered:
1980 orient red 240D 4-speed

Gone and NOT fondly remembered:
1982 Chna Blue 300TD

Other car in the stable:
2013 VW Jetta Sportwagen TDI / 6-speed MT