OK, it looks to be your safety relief valive. I would drain the air tanks with engine off, then see if you can get to it and work some fine oil in there. It is probably got some grit in there holding it open. Need to find a maintenance TM and see if it is mentioned as a maintenance item.
Your reference diagram, if I got this correct, is item 28, figure 113, Tech manual 9-2320-386-24p
It should not be leaking down there. Another thing I would want to do it, crank up the engine, let the air come up to full pressure, pump the brakes a few times, let it come back up to pressure again, shut down the engine with emergency brake on, climb underneath and see if it is still leaking with all systems shut down. If so, it needs be cleaned or replaced.
It is sold as a single unit. I cannot find any kind of reference to it being a maintenance item, but there might be something out there I have not located yet. For now, download the above mentioned tech manual from Jatonka.com and look at the above referenced drawing, and you will see what I am seeing. Good luck.