1986 A/C-Heater air flow, not right

[KenIDI93]

1986 F-250 6.9:
A/C - Heater control on the dadh no longer allows air to defrost/floor/mix (maybdodoe mix?) I don't use A/C (disconnected the belt).
All was working as it's suppoed to. Then last week, when moving the arm/lever for defrost/mix/floow/vent, all that would come out was air blowing on each side of the dash an next to the glove box! Not good, especially with cold weather approaching.
I bought a Motorcraft A/C - Heatrer Valve (E1AZ-19B999-B, small town Montana Ford dealership had one in stock - since 2010).
Did the change out of that part (oh what a wonderful time that was) and no change to the condition of only blowing air on each side of the dash and next to glovebox.
Most likely some sort of vacuumn issue?
There is a small vacuumn tank(?) to the left of the glovebox. I'm not familiar with all it does, and of course there are vacuumn lines to be scouted out.
Any suggestions would be most helpful.
I'm no major mechanic, but do some and always willing to dig in n' give things a try.
thank you...

 

[BlindAmbition]

Not sure your exact issue, but I've rebuilt my entire AC/Heater system and can tell you the issues I found.

The blend door itself had hinges that were damaged, so I made my own hinges. The rear of the heater/AC control panel had a rubber block that all of the vacuum lines attached to which was then plugged into a vacuum switch of sorts. This switch leaked, I went to the junkyard and robbed 2-3 and the first one functioned for me. Past that it was just a couple of general vacuum leaks from old lines.

Circled the offending switch in blue, it is the tan piece that the rubber vacuum line connector plugs into. Some of the prongs had cracked, probably from less than careful handling on my part.

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[KenIDI93]

Thank you for the input/suggestions.
Now to scout out pieces n' parts...

 

[Noiseydiesel]

I'll throw in my 2 cents worth. Typically if the HVAC vacuum hose fails, the default position for the vents is the defrost mode.
If you have air coming out of other vents then I suspect your control is broken. As for the hard plastic lines, you can repair those easily enough with rubber windshield washer hose. Get the smaller inner diameter hose, wet the hard plastic and the inside of the hose and slide it together. If you don't have to force it, your hose is too big.
Wet, ie, spit.

 

[franklin2]

Here are some diagrams and info on a 86 system.

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[franklin2]

Electrical part of it.

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[franklin2]

Function description and legend for above electrical diagram.

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[franklin2]

How to troubleshoot vacuum circuits.

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