If you don't have a Mity Vac, put a long piece of vacuum hose on the valve in place of the brown vacuum line and suck on it. If it shuts the engine down, the valve is ok and the leak is elsewhere. This is the most common situation.
Shut off valve won't look quite like that on the car. It's hard to see, down between the back of the injection pump and the front of the oil filter housing.

Depending on the amount of redneck in your genes, you could simply run the vacuum hose out of the engine compartment at the cowling, leaving about a foor or so protruding. You can then shut off the engine without opening the hood by simply giving two quick sucks on the line.
If your redneck heritage is less, you could take the time to thread the vacuum hose thru the firewall leaving it accessible to suck in the passenger compartment.
If you have a long dignified family line, you'll have your footman seek out the system's vacuum leak and repair it.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13