I seem to remember the doors being open with no vac, but I'm no longer sure.

There is a blend door (the one inside the box) that might cause trouble by letting air through the heater core, but I'm not sure.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!