Interesting feature
 

With A/C on, and air coming through the center vent if you nail the throttle (so as to create more vacuum) the air momentarily re-routes to the defrost vent. As soon as you let off, it resumes coming out the center.

Any idea what this clue means? (OK, here come the wisecracks....like, "it means you're blowing through too much gas..")

What I'd really like to know is what part may be failing that allows this?

Open throttle = less vacuum, and the climate control doesn't like it :-)

Look for a black/white check valve in one of the vacuum hoses near the firewall. I can't remember exactly where....been to long :-)

Cheers
DD

As usual Doug, I have it bass-ackwards, thanks for straightening me out on the vacuum. I'll look for the hoozit.