It's an 82 n/a. The dash is out and we're trying to figure this out. We found some vacuum lines that weren't hooked up, so we hooked them up, some cracked ones, so we replaced them. Everything seems to be hooked up right. Now, when the unit is turned on, the plenum door will open on the bi setting only, but eventually will fall closed like a lack of vacuum. Everything else just blows through the floor vents. Cold air will come through the plenum door, but hot through the floor. I know it all works off of vacuum, but I can't figure this out. Does the control unit go bad? If so, is there someone that refurbishes them or sells new ones? It's one of our customer's cars, or I would recommend swapping to a manual unit, but he wants it "all original".

 

The "control unit" is just a bunch of ports on a disc attached to the vent selector lever. It opens and closes ports as the disc moves over the vacuum lines.

If the ports are closing back to default, yes you still have a vacuum leak. It's easier to diagnose problems on the bench... so if you have time, remove the center control box and do the tests on the bench.