I have an 88 G20 that's I've been working on fixing up. The current problem with HVAC that I can't quite figure out, is that it won't switch to floor for heat. If I remove the round vacuum selector on the back of the controls, I can run the vacuum line directly and will then switch to floor just fine(Or any position I want manually). But when the vacuum selector is in place, on Heat/Blend/Def I can hear a vacuum hiss, and it doesn't properly switch. Everything to the left of Heat seems to be ok.

I've taken the vacuum selector apart, and it looks good. I've tried some silicone grease to help it. I've tried another one, and I get the same result.

Any idea on what is going on here?

 

usually broken vacuum supply lines. The heat under the hood breaks them down. just replace them all and the checkvalve too. Leak test the vacuum reservoir.

 

As I said, if I bypass the selector valve, everything works perfect and switches. So how can it be vacuum lines? Also, I've replace all the under hood lines that I could find.

 

Two bad selectors I guess. They are 35 years old and rubber doesn't last very long.
You could test them with a hand pump, or try a control unit out of a newer vehicle