Leak at Air Compressor

I have a nice size air leak at the air compressor. Tru Flo 750.
The leak is on the inboard side of the head, the side of the head that faces the engine. If you open the chassis battery compartment side door, put your hand over the top of the compressor head you will feel a round object about the size of a 50 cent piece in diameter and about 1 inch deep. It is screwed into the head and is brass in color and has a large hex shape built into it. There are two small holes in this gadget that are 180 degreees apart. Air is coming out the holes.
Any idea of what this piece is, what may cause this problem, and how to fix it?
Picture is the compressor head.
Item in question is located about where the hose clamp screw attached to the blue hose is positioned.

Spoke to Detroit Diesel and am told the leak I am hearing is coming from the Pop Off Valve. As long as the system is building good air pressure, the recommendation is to leave it alone. Requires the removal of the aircompressor to replace the Pop Off Valve.