My 86 n\a was doing the same thing. I repaired the vacuum lines under the hood going to a container right in front of the radiator resivoir. Now I've got ac and no more nagging hot air.

 

My 88 turbo was having a similar problem and I heard a hissing from under the dash. I removed the panel under the steering wheel (3 screw) and checked the small manifold where there are six solenoids and a lot of hoses. Three of which go to the actuators that work the different doors above. Then as I looked around I found a hose split that had been cut and coupled with one of those plastic couplings with the wings on them that you purchase at an auto store so i cut the hose back further and re inserted it onto the coupling and all is well now, I have A/C coming from the upper vents as it should be.
You might want to check this area by starting your engine and listening for a hissing sound under the dash.