I've replaced an accelerator pedal on a 126. I had no problems with high idle. That could be your issue right there. Fix that problem and report back.
You can test your vacuum shut off diaphragm by replacing the brown line with a piece of vacuum hose and sucking on it with the engine running. If the engine shuts off and you don't get a mouthful of oil, your shut off valve is working correctly and the vacuum problem is elsewhere.
It's not an unusual problem.

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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