2 stage compressor question

[1BAD68]

Yesterday my compressor stopped pumping at 30 psi.
It would run fine and didn't make any strange noises but just wouldn't build more than 30 psi.
After a couple hours checking valves, rings, pistons etc. I decided to unhook the line shown in the picture and see if it was plugged.
I have no idea what its for but I installed a plug in the head where the line hooks up and now my compressor works fine again.

Does anyone know what that line is for and could explain its function?

[John_Kunkel]

That line leads to a centrifugal unloader.

When the compressor pumps up to it's rated pressure and shuts off, the centrifugal valve in the crankcase opens and dumps the trapped pressure in the pump secondary cylinder to unload it. (you often here the short burst of air escaping immediately after shutdown as the pump comes to a halt); that way when the compressor restarts the motor doesn't have work against trapped pressure in the pump cylinder. After the compressor comes up to speed the centrifugal valve closes and allows pressure to build in the cylinders.

The unloader valve is probably stuck in the open position and won't allow pressure to build. Disassemble the end of the pump and look for the cause.

[1BAD68]

Thank you for the info 

[1BAD68]

I took it apart and found that there is a small rod on the end of the crankshaft that works like a plunger and goes into a oil pump of some sort.
When the crankshaft starts spinning it moves the rod up and down pumping oil pressure behind a piston in the oil pump which then moves outward
and presses on a lever that shuts a valve preventing air from going through the line that I disconnected.
Then when it shuts down, the oil pressure releases and the piston returns opening the valve and releasing the air.
There is a copper screen that filters the oil in the little oil pump, it was plugged with sludge.
I found about 1/2" of sludge on the bottom of the crankcase so I cleaned it all out and now it works great again.
While it was apart I could see the cylinder walls and they look like brand new with crosshatching still visible.
Its amazing how they engineered these old compressors and built them to last.