ac vent controller on my 1984 Chevy Pickup

I noticed the other day that when I switched from Max AC to vent to floor to defrost the air will only blow out the floor vents. Is this a common problem with an easy fix. Anyone else experience this problem?

Check for a vacum line leak under the hood, probably on the passenger side. 84 should have a vacum booster "ball" check to make sure the vacum lines going to it are good

I failed to mention that this has a diesel engine. Will this make a difference because I could not see what you are talking about.

It makes a differance on vacuum source.
They have a vacuum pump on them.
Check it for pulling good vacuum.
No vacuum and the ac/heater will not work right.
MT