Iveco 75e15 air brake system ?

Having owned my 75e15 for a number of years now I have had to replace the unloader/bleed valve a few times due to them gumming up and sticking open/closed. Im having a similar issue which again I thought it was the unloader valve, having replaced it again with another 2nd hand one Im still experiencing the same issue, my tanks keep over filling. Now today I found a random air line along the chassis rail right below the oil filters that isnt attached. It has a push style connector on it so obviously there was a air line attached or should be but I can see no other end to this line to re-attach it. Now this air line only seems to let air escape once the tanks hit their 12bar designated pressure, Im guessing whatever this air line it is something to do with the tanks regulating themselves, as below 12bar this air line has nothing coming out of it.

Does anyone have any sort of schematics on the air system of a 75e15, I have tried googling but cant seem to find anything ?