sounds like a vacuum line is hooked up wrong. i had the same happen to me but my front windshield on the 77 is out so I wasnt locked out. Check the vacuum line config at the mast vacuum switch in the drivers side door

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)