I read your original question. I did not read the next 10 pages of replies though. Before you pull everything apart, might want to check that the blend door (directly above where front passengerís feet hit firewall). That flapper door sometimes gets blocked by leaves or debris. Next, you might want to check out my post in this related thread covering vacuum hoses and reservoir.
In your case, with gas engine, you need the vacuum reservoir and related hoses all intact and airtight. The hoses may have cracked near connections to vacuum reservoir. It is important to note that with a gas engine, you cannot delete the vacuum reservoir like you can on a diesel. It holds some vacuum between trips. Easiest way to reconnect new hoses (if necessary) is to cut an access hole (at 12:00 above front right tire at top of inner fender). The reservoir on all Ford vans is between inner fender and fender. If that is necessary, make a coverplate for hole and add your favorite undercoating afterward. Hope this helps. Cheers!