I have the *exact* same problem. Foot vents - perfect, defrost vents - perfect....face vents - nothing.
Grateful for any pointers you can give me (inc parts to order that I might need). Plan to try and trace the lines at some point, but it's 100 degrees here right now and don't want to be outside or inside a hot van!

 

If it's too hot finding a big shade tree or waiting until nighttime will help. The vacuum canister is under the passenger headlight and the vacuum hose from it runs to the middle passenger side of the intake manifold. At the manifold there is also a one-way check valve. You should also check that to make sure air only passes through it one way. If you remove it, note the direction so as to reinstall it the correct way.

 

thank you for the tips. I'll be getting to it this week I hope.

 

so, despite the 100-degree California heat, I decided to tackle the issue with the air vents and I think I found the issue...I think
Looking at the pics I took, there is a grey vacuum line that connects to the vacuum control valve (not sure what to call that metal housing)? that isn't connected to anything. It's super brittle and snaps if you so much as look at it. Any idea where it should actually connect to?



red line on the side, black and grey connected together and into the end (looks like this is how they should be due to the matching connector)



grey line passes through the firewall


comes up from the firewall and goes.....nowhere

Judging by the tape around it, I'm guessing the previous owner didn't know where it went either and simply taped it up. If I can figure out where it goes, and find a place that supplies replacement line, I might be able to survive while driving the van!

 

I should point out that the operation from heat to cold works (can see the level moving when I turn the dial), but I haven't yet tried to move the diverter valve for the air flow. I suppose I should try that to make sure it's not siezed.

 

aha...I may have found the issue....or not
In my earlier post, I showed an image of a grey vacuum line that goes nowhere because it has snapped or become disconnected. In a Facebook post, someone advised that the grey line goes to this unit (see first pic)....but...see second pic...I don't seem to have that unit. The two hoses have been joined together via a connecter.

So
1) what is that unit called
2) any ideas where I might obtain one (assuming they're discontinued)
3)....er...nope..that's it I think. If I can get one and it fixes my issue...all good.

 

That has to do with coolant flow to the heater core. Just plug the line and see what happens.
Usually that vacuum diaphragm is located further back and it looks like you have a splice there as well.

 

I've compared my engine bay and components to that of others that have sent pics.

 

I have the valve on the left. Maybe I missed it, but I'm guessing you have rear heat.

 

yes, I have rear heat also (conversion van)