Rear AC Blend Door Vacuum Source
 

2005 E-350, 12 passenger/5.4. Bought the van knowing the rear AC would automatically switch to heat while driving it. I thought it would be the blend door, but low and behold, the blend door/vacuum canister work fine. The problem is that while driving the vacuum going to the canister drops off to a point where it switches back to heat. To accompany this, the front dash switches to defrost while driving as well, not switching back to vent's until the engine idles. Brakes are fine. I have not actually put a vacuum gauge to see if the engine is having a problem itself, but because its driving fine I'm leaning more towards old vacuum hoses/fitting leaking on their way to the dash/rear.

So, after I test and confirm the engine is making the correct vacuum, I'd like to fix the vacuum issues at the dash and rear. I've seen the video's on how to DIY the dash vacuum issue, and was wondering if I could do the same for the rear as well. But if I give the rear a constant source of vacuum, to me that would mean it would always be on the AC side of it. So the vacuum line going to the rear must get a vacuum source switched to it when the panel is switched from heat to AC, correct? If so, where does this vacuum source/switch come from/occur? I can see the vacuum line follows a the wiring harness, but since I haven't taken off more panels then needed to access the blend door, I can't see where it goes.

Any help would be appreciated.

Alright, I crawled around under the dash to see whats up. It appears that the common failure on this side of the vacuum system is the vacuum resevoir failing. So I found the lines that went to that (thanks youtube) and then followed the vacuum lines to the top of the dash where there are 3 vacuum actuators, with one stray blue vacuum line going back into the dash. I found that line going to the wiring harness in the drivers side footwell, and there it makes it back to the rear heat/AC setup. So, I'm going to go ahead and replace that vacuum reservoir/fitting with a new one, to see if that helps remedy it.

Your plan to replace the vacuum reservoir is solid---that's most likely the cause of your issues, at least in my actual experience.

You can buy the OEM reservoir and check valve for less than $30 and install it all inside the cabin. Trying to find and replace the reservoir is NOT easy and for the most part simply impossible. I have a thread here entitled "Finally Found It!" or something to that effect showing where its located---seeing that you'll understand why installing it any place other than the factory location is really the only option.