Vacuum operated heater air conditioner controls

[fnny6770]

This is on my 1980 GMC k 3500 truck, Look at the attached picture, this circular cup looking device on the air conditioner box, it has two vacuum lines to it, the bigger one described with the arrow connects to manifold vacuum then the smaller diameter line pointed out with an arrow goes inside the cab up under the dash, my questions are why does this cup have to vacuum lines to it? And where does the vacuum line that controls the heater air conditioner controls in the cab come from?

 

[75gmck25]

The small diameter line goes through firewall, across the back of the dash, and then provides the vacuum source for the A/C controls. As you move the dash selector switch, it switches vacuum to the four (?) vacuum motors that operate the HVAC.

If you look at an original GM HVAC diagram it will usually have a color code for each small vacuum line, and the original lines will have a stripe of the same color.