What's the function of the brass piece along the water supply line. This is the supply line for a hydronic floor heating system. Something is malfunctioning and one of the possibilities is that I have an air pocket somewhere along the line. I'm wondering if this device is there to somehow release the air from the system.
As the "Spirovent" label should lead you to believe, yes, it's an automatic vent (albeit one that looks rather gunked up at the vent.)
This would appear to be the instructions for your device:http://www.spirotherm.com/sites/default/files/JR%20IOM-B.pdf
You should also look for manual vents along the loop at high points. This helps keep pockets from forming if working correctly, but does not remove the need for manual venting if the system was drained and refilled, had a leak, or otherwise has lots of air in it.