Try testing it the other way around. Connect the MityVac directly to the shutoff line and pump it by hand to shut down. If it won't hold vacuum, then you'll need to replace the shutoff mechanism. Be sure to read the old threads on that subject, so that you're aware of the possibility of destroying the engine.

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Rick Miley
2014 Tesla Model S
2018 Tesla Model 3
2017 Nissan LEAF
Former MB: 99 E300, 86 190E 2.3, 87 300E, 80 240D, 82 204D Euro
Chain Elongation References