my heater is only blowing air out of the defrost I know that means there is no vacuum going to the climate control but cant find a leak anywhere is there anything that I maybe over looking that someone could help me out with any help would be great thanks!!!!!

 

Out under the hood engine bay, make sure the hard plastic line by the heater motor and wiring harness is still connected to a vacuum source on intake.

And under the dash it may be hissing, Theres a thread on this just last week. SEARCH!
GL!

 

I've got the very same problem on my 85 z-28
I've searched for the leak, but still no luck... nothing behind the dash... so that's probably a very good thing, since the piece that malfunctions back there is no longer available.
I'm planning on diving in more into in a couple of days...
fingers crossed

 

The rotary switch has to have a very good seal to actually work properly, I've had mine out and swapped with other ones several times. After 20 years of sitting the rubber just doesn't seal well. I swear one of these days I'm just going to replace it with some solenoid system, that switch is just awful. Of course you have to actually have vacuum to the switch first.

 

https://www.thirdgen.org/forums/inte...anent-fix.html