I cannot imagine any way the vacuum system would know what gear the car is in. So I cannot answer why this would happen in L and 2, unless the vacuum line to the transmission modulator is somehow caught in the shift linkage. I can't visualize it, tho.

I would take a shot at an answer by saying that your motor mounts and/or transmission mount are worn enough that when you put it into gear the motor is moving enough to seal off whatever is leaking. That suggests to me a rubber connector or possibly even your main vacuum line to the brake booster.

Have you checked how much vacuum you are getting from the pump through the main line? That is a useful thing to do before anything else.

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Mac
2002 e320 4matic estate│1985 300d│1980 300td
Previous: 1979 & 1982 & 1983 300sd │ 1982 240d

“Let's take a drive into the middle of nowhere with a packet of Marlboro lights and talk about our lives.” ― Joseph Heller, Catch-22