venting system control valve

This is my first post on this forum, although I've done countless repairs just by reading archived threads. I can't even begin to say how valuable this resource has been to me.

My car is an '82 300TD. I have some questions about a plastic component that's vacuum controlled-- one vacuum line going into the top and one into the side near the bottom. From pictures and charts that I've seen, it's called the Venting System Control Valve.

My first question is, what does this part do?

Secondly, I've tested the compression on it, and the top is fine, but the plug on the side doesn't hold any vacuum at all. Is it supposed to be like this, or is it completely blown out?

My final question is, if it is blown out, does anybody know where I can get a replacement, because I can't seem to find it anywhere.

I'm attaching a picture to help clarify.

Thanks so much.