1980 Chevy Corvette Air conditioner will not blow cold air

I have a 1980 corvette, and no matter what I try I can not get the air conditioner to blow cold air once the temperature outside gets warm. My brother has a 1979 corvette and has the same problem. What is the solution to this problem. I have just had it checked out and the freon checked..it is full...but I can not get the thing to cool.

Thanks for the help
Mike