My vents are stuck on defrost. I’ve used a vacuum pump and found that connected to the hardline in the engine bay it will hold vacuum if the vent setting is off, but as soon as I move the switch to Max is loses vacuum. I’ve pulled the vacuum switch to check for the common problem of it being loose and it’s fine. Doesn’t seem to be leaking from the switch. Where else should I be looking?

 

Probably the vacuum motor (diaphragm) for that function.