Defrost doesn’t work

[munch]

I have a 1986 K 20. The heat goes to the floor fine and the vent changes as it should. When I swap it to defrost the door closes partially and I do not get the full air flow to the defrost. I checked the vacuum hoses and they seem fine. I can reach behind the glove box and actuate the door flap manually and it will go to full defrost but won’t stay there. Any ideas?

 

[Snoots]

You're not going to like this.
From the inside, remove the HVAC cover. Look inside to the left. You will likely find this part broken. You can fix it but it's likely to break again.
If it's the part that is broken, look here:

https://www.amazon.com/dp/B0B5WHZNGN/

 

[Grit dog]

I don’t even want to look at the broken part but the defrost doesn’t work in the 77.
Figure gotta take the heater box out at some point lol. Not the first time.

 

[gmbellew]

there is a panel that you can remove from the box under and to the left of the glove box. it is kind of a vertical piece, maybe 3 or 4 screws. you should be able to check for broken door clips and watch how the blend and air direction doors function and see what the issue is with it off