1980 450SLC Air condition problem
 

I have a 1980 450SLC and with the air condition turned on I only get air (sometimes) from the defrost and the two side vents. This makes no difference where the selector button is. Some times all I get is warm air out of the vents(50% of the time) Normal setting is Bi Level selection.
Any suggestions. How hard is it to check the vacuum switches behind the dash?
Rick