Let a shop do some hurricane repairs and the van
came back not-right for climate control. The airbox
would not send air to the right outlets (floor, dash,
defrost all going half-assed) and temperature was
likewise mezzo-mezzo.

Internet advice focused on the thin plastic hard vac
lines as embrittlement-prone, but showed there are
two trouble spots - the manifold vac port, and the
vac reservoir "ball" up by the battery.

In my case I found that the "ball" soft vac line had a
split in the end that goes to the vac "T" where the
hard line, the air box vac feed and the vac reservoir
meet. So there was a vac leak there, that the manifold
could not fully pull (there's a check / restrictor upstream
a couple of inches not to mention the restriction of the
hard-line run).

Probably just a casual bump knocked the hose off the
"T" and nobody noticed (aside from the wife who's very
fussy about temperature).

Cut off the split end, put it back together, all nice-nice and
a 2-minute fix. Didn't even have to take off the doghouse,
bonus. Always start with what you can get to easily.

 

Great advice. I am surprised as to how it acted. Usually, a lack of vacuum makes it revert to defrost (by design).

 

Maybe because of how the fault was, there was a loss
but not total loss of vacuum - whatever the feed run
could pull, against the "orifice" of the open "T" arm.

A/C was blowing out of all vents including defrost
(so actually fogging up windshield, near the vents,
at night).