I'm gonna put this reply in both my threads about this, so you might see double post....
I "fixed" my vacuum issue today with the outside vent. I was spending a day on the vehicles and decided I had the time to mess around with the lines. I found my problem about 20 seconds into it. I had started following the vacuum line from that black cannister thingie in the above pics, and the first one I couldn't plainly see where it went ended up going down into the engine about 10 inches and then it was cut. The cut looked like it had gotten caught between something and smashed in two. I got a flashlight and tried to find the other end, but I never could.
So I started the truck and sure enough, there was a vacuum leak right there. It wasn't enough to affect idle or anything, but I could hear and feel it at the hose. I tried to find a screw to screw into the hose to seal it (my goto move LOL), but I didn't have anything small enough. And I used to have a ton of vacuum caps, but I haven't seen them in years. I thought duct tape was a little overkill so some black electrical tape did the trick.
So I hooked everything back up inside, behind the glovebox, and it pulled the plunger down with quite a lot of force. I then turned off the truck, and it held the vacuum, I couldn't pull the plunger up. (I still had it removed at this point, just hooked to vacuum) So fixed it seems.
A couple of interesting points. It holds vacuum fine OUT of the truck and just hooked to vacuum from behind the glove box. But once I install it in the truck, it seems to slowly loose the vacuum. Not too fast, but maybe 5-10 mins? It seemed to be something about the angle the hose sits at once installed. When I was installing it it would hold vacuum fine until I started trying to get it back in place. Then it would lose all vacuum over a couple of second while I was moving it into place, and the hose has a pretty sharp turn right where it hooks up to the plunger thing. So I think there is a small leak that acts up sometimes right where the hose hooks to the plunger.
However after a test drive, it still holds more then enough vacuum to keep the vent closed while moving. So I'm calling it fixed and resolved. I did verify it ONLY closes the vent on OFF and MAX A/C..... All other settings the vent is open. I would rather have 100% control, but my other choice is 100% closed, so I'll play it Fords way. Really the only time it would matter would be on Floor Heat. There is not way to turn off the vent and have Floor Heat. You can just turn it to MAX A/C if you want Heat from the vents without outside air cooling it down. (it doesn't turn on the A/C, it's just the same setting as Panel except with the outside vents closed)
Oh, one other thing. I did track down the other vacuum lines from that "black canister" under the hood. Most lead into the firewall, but it is in fact "powered" from the vacuum octopus on the side of the intake.