Vent Control

Looking at the pics, that should pull right off along with the white line next to it. There may be some sort of retainer or clip on the back side that you can't see. If it were me, I'd pull somewhat hard on it and see what happens.

I'm not too familiar with the Fords, but the GMs work similarly. No vacuum puts all the air to the defroster. I'd give a good listen up under the dash to see if you can hear a whistle. Also, a hand vacuum pump might come in handy to test the line from the engine to the interior.

Thanks for the advice. I will go and get a hand vacuum pump tomorrow...

dangerruss wrote:
Thanks for the advice. I will go and get a hand vacuum pump tomorrow...

They come in handy if you bleed your own brakes, for testing vacuum advances (old school) and anything else where you need a vacuum source. I don't use mine often but I'm glad I have it.

I searched for just such a leak and eventually found that someone had replaced the battery and sat the new battery on top of the line which eventually wore a hole through it.
SO maybe look for anything that has been touched since it was new.

Before you go any further just take an unlit propane torch and slowly run the nozzle along the vacuum lines and fittings with the engine idling.

When you get to the leak the engine will inhale the propane and speed up a little.

It makes tracing leaks really easy, you don’t need much light and can get into places your hand might not fit.

https://www.youtube.com/watch?v=34t9uaNnTRQ

Does this happen ALL the time......or just when the engine is pulling hard ??

My Ford V10 would lose enough vacuum when pulling a hill to do that; then went back to normal once over the hill. It started doing that after applying a "custom tune" and stopped doing it after I went back to the stock tune.

If it is a full time problem, the trouble can be at or close to the controls too.

Mentioning what kind of engine you are working with might help.

I had this problem and found there's a vacuum reserve tank and a vacuum check valve BURIED inside the HVAC Cabinet. Solution was to bypeass those parts by installing a new tank under the hood along with a vacuum tank near it. Then ran new hose to the interior. It's all documented HERE.
Diesel doesn't produce engine vacuum the way a gasser does, but I recall researching this for another member and it seemed the diesel uses an external pump to produce vacuum for things like HVAC control.
By all means, inspect under the hood. I had a broken vacuum line on another Ford caused by installing a battery too. But if not...
My "cut to the chase" is this: in this pic, you see where the black hose comes out of the HVAC cabinet in the toe space of the passenger side foot well. . You can see the black hose cut off where it comes out of the black cabinet and highlighted against that sliver of white. At least on a gasser, I would cut that hose, start the engine, and see if there's vacuum on the cut stub. There will probably NOT be vacuum there.
I wasted a lot of time pulling the dash apart and don't want you to. Will help any way I can. You can PM me a phone or email ID if you want.

If you look closely at the the black and white lines it looks like there is a ring at the end of the coupling where the lines goes in, if so push that in and the line will come out. When re-inserting just push the lines into the coupling and they will lock.