The check valve on the AC system supply vacuum line is bad. This will drop the vac when manifold vac drops at wide open throttle, so the flaps all close and you have defrost only.

The other possibility is vac leak in the AC system itself.

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!