Old Curtis Compressor Education Needed

[mrtonegto]

I see that many of you folks have these old compressors from the 40'-50's.
I'm working on getting my Curtis model 95 up and running and frankly I don't know much about the workings of them.
What is the purpose of the copper line that runs from very the top of the unit down and into the crankcase cover?
Is that line under pressure? or vacuum?
Looking to learn as much as I can on this old boy................

Tone

 

[b-body-bob]

That's probably going to be your unloader, that lets off the head pressure when the pump stops turning. It will be under pressure when the pump is turning.

Post photos.

 

[mrtonegto]

Here's a couple:

 

[mrtonegto]

BTW, u can see the pressure relief valve just laying there on the unit. It broke off and belongs on the fitting to the left of it. It's since been installed.