At the front of the engine just inboard of the power steering pulley is the vacuum pump. It has two connections - a big pipe to the brake booster and a smaller vacuum hose. 3-4" up from the connection is a small plastic filter. I think it's yellow. That's where the bad screen is.

If that's not the problem, you probably have a bad diaphragm in the center vent acutator.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD