I bought a new A/C control from Modern Day Muscle(good service; prompt delivery) and when I installed it, the a/c would only blow out the vents on the top of the dash pad.
When I started the car, I could hear a hissing from the dash- vacuum leak! The controls that direct the airflow to the dash, floor or defrost are vacuum controlled, so I pulled the unit out carefully for a look... when I pulled the vacuum lines off the back of the unit and started the car, I could silence the hissing by covering the lines with my finger tips, so there was the problem.
I lubed the lines with a little silicon sealer, reinstalled them( don't forget the two spring washers that hold the quickconnect for the lines to the control, otherwise it will leak again) and voila!
It works like new and all is right with the world again!
By the way, I had been smelling a burnt wire smell for a while and when I pulled the old unit, the quickconnect that plugs all 5 wires into the blower switch had shorted. This quckconnect is discontinued, but it is easy to splice on some connecters and plug each wire on individually- the quick connect was just to speed assembly on the line when these were manufactured, so Don't freak if it happens to you.