Maybe the check valve or vacuum motor diaphragm is leaking?
I do recall that Dave had patched the bottom of his vacuum reservoir with fiberglass.
Maybe it is starting to lose its grip on the metal can?
I'm sure those things flex when vacuum is pulled on them.
Not sure why having it blow out the defroster for a minute would make much difference overall.
But I don't have
any AC. It can get pretty oppressive when I'm not moving.
Jim,
Lil'Red is a '87 F250 HD, 4.10's, 1356 4x4, Zf-5, 3G, PMGR, Saginaw PS, desmogged with a Holley 80508 and Performer intake.
Too much other stuff to mention.