I started out with the ol' finger blocking test. You should be able to block the output of the ball, or pump and the motor will shut off. I just happen to have a vacuum pump/ gauge checker from a previous MH issue I was chasing, so it was handy to have. But this pump (as far as I know) only supplies vacuum to the dash HVAC solenoids. Unless it's open between the ball and dash control (or the control itself is in trouble) you should be able to get the pump to shut off in some position: Def, Mix, Vent, etc.
As flakey as those little tubes have been, look very closely at all of them because this seems like the norm rather than the exception.
Jeff - WA6EQU
'06 Itasca Meridian 34H, CAT C7/350