Vacuum switch on 1985 300D

[3eagles]

I've been tracking a Climate Control issue with my 300D. During the check of vacuum lines I found a small black line leading behind the brake booster was not connected. Behind the booster there is a small switch with an electric two wire connector at the bottom. It has a small tube that sticks straight out from it and an odd looking little cap on the top. I used a piece of spare tubing as a sleeve and connected the loose black hose to it. I didn't see any change.
Question is, what the heck is that switch/valve and what does it do?