as i have had several early caravans, that problem surfaced on them as well.
it was in the control, plus a check valve.
i know you don't have a caravan, but are experiencing the same problem with a vacuum control as i did.
you may find something if you pull the control from the dash and check out the back side of it carefully, as well as the connecting hose assembly.
just my experience with the same problem, although on a different vehicle. actually, several in the 85-90 something year range.