I certainly remember some flaps moving when I tested the AC in 1983 240D with manual AC. There is also a lever on the center vents, I think (maybe not) that closes off the outside air. There is on the W115 chassis, but that doesn't mean much.
You should be able to get unconditioned air out of the vents, and if nothing comes out unless the AC is on, I'd suspect the vacuum system. Check to make sure the green plastic line up by the brake booster isn't loose, then get a MitiVac or similar hand pump. Very difficult to apply that much vac by mouth (and not recommended!), but you can also use a pump intended for AC evacuation -- it will be too strong, but at least will let you know if the sytem holds vac. If it leaks down, you will have to look under the dash for the leak. If it works with external vac, you need to find out why you don't have enough.
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!