Sorry if this is in the wrong spot but I didnt see a a/c and heating section.. car is 92 camaro, ive been building the car for 2 years now. Now im doing the a/c it blows cold but only from defroster. Question is what is the black ball with the vacuum line coming out of it on the drivers side under the battery tray. I know the battery is on the drivers side just the best I can think of to describe it. Some say its for cruise control some say its for a/c.. second thing is does our cars have a check valve by the firewall as the lt1 does with the 3 vacuum lines going to it for the a/c. I dont think so but figured id ask. Last thing im going to look at it the control unit. If the black ball is ac related then thats my problem. Because I removed it...

 

That vacuum reservoir needs to be in place for the HVAC controls, and the hoses (interior and exterior) need to be intact. DEFROST is the default position when there is no vacuum.