I had a similar problem in my w124. Firs I thougt it was something related with those vacum pods but then I noticed I had more vacum problems (not only central vents operation). In my case, the rear head reast were not deploying. Finally, my mechanic found a disconected vacum hose behind the engine block, close to the oil filter. Looks like someone accidentally disconected it when doing a filter change. It was an efective and free fix.

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Juan
1995 E320 Sedan
2008 Ford Escape
RC Helicopter