I think we're having the same problem, Oil Leak ?
If the line your talking about exits the back of the housing, below a solinoid and behind the compressor head, changes through a 45 degree fitting then goes to the side of the cylinder head.
My oil leak started on the road , I picked up a 19mm open end at a yard sale then ground it to fit, tightening the nut stopped ( slowed ) the leak for my trip home. Toad is a mess.
I believe the line is a high pressure fuel supply to the head, and I'm looking into what it would take to remove the solinoid for better access or taking the rest of the tube off to get at the nut with a deep socket.
I'll tag along here to see what answers you get and keep you posted on everything I get for info. A friends son works at a Cat industrial shop so I have him looking for manuals .