Check the vacuum line (green) in the engine compartment near the brake booster. Most likely there is a leak, usually rock hard rubber connectors that look like vac hose, and can be replaced with generic vac line from the auto parts store.
If the vents close on acceleration, it's almost definitely a leak unless you also have a bad running problem resulting in very low vac, and even then there is usually enough in the reservior to "coast" the AC over low vac condition.
Check all the rest of the vac lines at the same time, they are all probably in bad shape. Check the idle control valve hoses especially in the M103 and M102 engines, they will be rock hard and probably leaking -- causes throttle lag and rough idle.
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!