Originally Posted by sdsknifeworks
. . . And the tail end goes up into the wall of the bus about 1/3 of the way back. . . .It's possible it's a vacuum of some kind but I don't know to what. . . . SDS
The photos of what the hose connected to were not clear enough to mean anything to me at first. Looking at it again with the extra clues, I noticed there are no hose clamps to keep the lines from BLOWING off, so vacuum SUCTION makes sense. When you say it goes into the wall of the bus, I am assuming you mean the passengers' compartment.
Many heater/air conditioners use vacuum motors to move the vanes in the air boxes to select heated or outside (cool), outside or recirculated intake air, or to defrost the windshield versus to heat your toes.
I don't know the vehicle model, and can't be sure, but since it appears to be on/off (all or nothing), I would now suspect that it might probably control suction to move an air vane to select recirculated air if there is either a recirculation switch or "MAX AC" position on the heater controls.
I also now remember that in some of the vehicles I have driven in the past (and I drove two Chevy vans), at start-up I would rev the engine a little while idling and then snap my foot off of the pedal to boost the vacuum to get the heat controls to change to my new selections more smartly. I don't remember doing that lately driving SUVs, but it also may be automatic and not conscious if the heater response is slow at this point.