Vacuum Sphere of Mystery?

[Supercharged111]

The factory included these reservoirs so vacuum actuated accessories wouldn't stop working on a long climb in a nutless car that was basically WOT to lug its own weight up said hill. Without your HVAC would default to whatever spring loaded position it was set up for.

 

[69hurstolds]

Single port ball is for cruise, two port ball is for hvac controls to switch vent modes.
The one pictured is for cruise. You need a manifold vacuum source with a check valve for it to work properly.

Depends on the model and options. I know 84-87 VIN 9 cars have a large single port ball for HVAC and cruise.