My 79 300SD has a shut off problem. In tracing the vacuum lines, it shuts off fine when I disconnect and plug the line going to the tranny. I guess that means there is a leak in the vacuum modulator. Could that cause the tranny to slip in high gear?

One other question: When testing the functioning of the shut off valve, I noticed that I could shut it off with my vacuum pump, but it took a lot of pumping and it was not building vacuum very well. Is this a problem?

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Richard

"Die Schadenfreude ist die reinste Freude"

It does sound like there is an issue with the modulator, or at least the vacuum line going to it which is a likely possibility. I would do a pressure test on the modulator by connecting the MityVac to it at the fender and pull 15psi and see if it holds it. If it doesn't, first I would replace the rubber hose connector on the end by the modulator. If this doesn't fix it, then I think you need a new modulator.
Regarding the other problem with the tranny slipping in high gear, I would disconnect the modulator vacuum hose and plug the end you took off and take it for a ride to see if it still slips in high gear.
The modulator may cause it to slip when initially going into gear, but not likely to affect it when already in gear unless its adjusted way too soft. Try adjusting the modulator for firmer.
Does it take a while for your tranny to engage in reverse? That is a sign of a worn tranny, as could be the fact that it slips in high gear.
Have fun and good luck with it, Adam Bush