I would be looking for (as 4.3XJ says) from the rear half of the intake manifold...a 3/8" hose out of a brass nipple that runs from there to the vacuum reservoir. That is the ONLY vacuum source for the HVAC and the cruise control. Then check all the lines to and from the vacuum reservoir all the way up to the firewall.

Replace any hoses and/ or check valves if they are at all compromised (chafed, cracked etc.). Don't tape up the vac lines...Just buy new bulk vac line...it's cheap. The vac lines under the battery tray are the usual suspects.