Well, I delved into the dash this afternoon and found the vacuum block mounted vertically just to the left of the glove compartment. It has a vacuum source on the top and 7 vac lines attached along one side with an electrical plug near the bottom. Probably 1"X1"X6". I removed each line individually and when I applied source vacuum to them directly the respective actuators seemed to work fine. I checked the records on the car and this part (vac block) was replaced about 9 months ago. My original complaint was the air from the center vents seemed dependent on total vacuum. In other words, it would cease under acceleration and return as I leveled out at speed. As time has gone on, there is no air at the center and the defrost vents are blowing like crazy. This has gotten worse gradually and the overall impression is that the flappers are doing as they darn well please. The question I'm getting to is: Can the failure of the vacuum block cause all this and what are the odds of it failing again so soon? (The original failure was marked by no air coming out of the footwell vents while in heat mode during the winter.) Also, the vacuum block was quite warm to the touch during operation. Thanks to all for a great site. I've learned more in the few days since I discovered this forum than in my many prior years of Benz ownership.