Since your asking for heat out of the AC/Heat system your using more than one vacuum element. Asking for heat CLOSES the center vents. Your fresh air vent is open, your floor vents are open, If that button is selected. so you need to check all your elements for a leak. and the vacuum switches behind the Ac control panel for leaks.

See the attahed diagram for the flap logic when using the Ac/heat system.

Dave

Edited for key to letters.

Attached Thumbnails

   

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car