Dash vent - AC and Heat issue...

Why doesn't the forum code for posted pics default to width=600?

The lack of vacuum and going to defrost is not a safety design, it is just that all other functions require a vacuum
1. The BEST way to fix this problem and it is COMMON to Ford Chassis is to install a 12 volt Vacuum motor. This is the same motor Diesel motorhomes use since Diesels do not have a vacuum source up front.
2. The 2nd way is to install a 2nd Vacuum Reservoir canister to double your stored vacuum.
You problem is, on hi RPM loads on the Ford engine, you deplete all the vacuum in the Reservoir. Since the Engine is NOT generating vacuum, once the vacuum is lost the controls close/open the various doors on the Evap box and the air blows out the defrost. Doug

PS, all Motorhome Dash Vacuum controls have inherent small leaks. The motors and such will bleed a little and you cannot make them completely "tight".

https://www.partsgeek.com/gbproducts/MC/5337-07017433.html?utm_content=MC&utm_term=1999-2007+Ford+F250+Super+Duty+Vacuum+Pump+Dorman+904-214+99-07+Ford+Vacuum+Pump+2005+2004&fp=pp&gbm=a&utm_source=google&utm_medium=ff&utm_campaign=PartsGeek+Google+Base&gclid=EAIaIQobChMIo7y9p_q-2QIVQmt-Ch1J4AYFEAQYASABEgLTMPD_BwE&ad=47362750812

"You have a bad check valve and vacuum canister for the A/C blend doors.
I replaced mine recently.
I cut a hole in the inner fender liner to access the parts.Otherwise, the dash would have to be removed."

I remember that post, and never could picture where you made that cut. Actually looked under ours and couldn't ID a site. On ours (2002) the black tubing from under the hood (connects to red tubing from Engine near the Oil Filler) disappeared into the underhood side of HVAC cabinet. Then exited that Cabinet in the passenger side foot well.

"I have this "problem" with my 2001 Ford gasser. When under high load (like climbing a hill, the dash a/c cuts out (temporarily).
It works off vacuum and apparently, the vacuum canister is not that large or large enough. While I find it little annoying, it is not annoying enough for me to fix it."

Canister has the capacity IF there are no leaks. By "Leak" I mean either an External Leak (cracked Canister, Hose off, Hose Cracked, etc.) or an Internal Leak (Vaccum Check Valve no longer holding vacuum stored in Canister before Engine went into Load and Vacuum fell).

As designed and installed, it works. It just won't tolerate any leaks.

Quite frankly guys, Home Skillet has done one version of the work. I've done it too, with a LOT of wasted time before I came up with the fix I made, bypassing HVAC cabinet and fender well.

An auto electric guy told me he didn't struggle with a lot of tracing and probing. If something needing power wasn't getting it, he ran a new wire. I took that to heart. Once I got my head around the issue, the fix was easy. If I'd had what I wrote for you guys (and I can find that thread) the job would have been a couple hours max, total cost around $25.

I have this "problem" with my 2001 Ford gasser. When under high load (like climbing a hill, the dash a/c cuts out (temporairily).

It works off vacuum and apparently, the vacuum canister is not that large or large enough. While I find it little annoying, it is not annoying enough for me to fix it.

You have a bad check valve and vacuum canister for the A/C blend doors.

I replaced mine recently.
I cut a hole in the inner fender liner to access the parts.Otherwise, the dash would have to be removed.

I fixed this exact failure mode on a GM car a while back. What I found was a hole worn in the vacuum line going to the vacuum canister where someone had sat a new battery on it.
Find the canister and follow its line backwards.

Did you perhaps ask about this before? Default, for control system failure, is Defrost. Spring loaded toward that mode.

To run on level ground, you probably could run hose right from engine to vacuum controls and have it work. But on acceleration or climb, that would be inadequate. Hence, a little Vacuum Tank and a Check Valve to maintain vacuum till you need it. Problem on Ford E-Series, and many other Fords, is Valve and Tank are buried in the Heat-A/C console. Easiest, least expensive fix it to bypass the HVAC cabinet with new Valve, New Tank, and different Hose Routing.

I repaired our 2002 E450 by installing a combination Tee and Check Valve under the hood with a new Reserve Tank under the hood. From there, ran a new hose from there to the vacuum control "switch" valve in the dash. It worked well for awhile then didn't consistently stay out of Defrost when the engine was under load.

In my case, my new Check Valve was bad. Easy fix, OK since. In your case, could be anything. Loose or cracked connection or connector, hose, vacuum canister...?

Let us know what you did/had done to fix it, link to a previous thread (if that memory of mine is right and it WAS you) and we can try to help.