It seems that it is only a matter of time and the vacuum bellows will fail. No matter what setting one puts the switch on the climate control the air can be heard blowing, but it won’t come out the vents, or defrost or even floor. Additionally when it does work using randomly, using the turbo will cause it to drop away from the vents, so it is tied to a loss of vacuum pressure. I have three 780s all with the same issue.
I have replace the check values which is the source of the vacuum coming off the intake manifold (on the B230FT)and then as the vacuum line passes through the firewall it goes to a box that splits the vacuum out to four different bellows. I know these fail as I had a similar problem back in 1995 when my car was only 5 years old. We had to cut into the side of the airbox on the driver’s side and replace the bellow. It worked.
The problem is that I am not able to find a source for new ones. Old ones are going to fail in a matter of time. Maybe there is a way to pull them out, seal or rework the flexible rubber, but it doesn’t seem like an ideal solution. Anyone replace these? Any idea where to get them? There are 3 different ones with the following part numbers: