Hydraulic leak or vent hose on backhoe?

I have a quick attach backhoe on a Cat 931B. I just replaced the quick coupler hose that connects the incoming oil to the hoe valve because it had a small leak. It only leaked when it had a lot of pressure, which is good in a way, because I was still able to use the hyd.'s to remove the hoe, so I could repair the hose. If it would have been a major leak, I don't know how I would have replaced the hose. Anyway, I fixed the hose and added some oil and I was digging for about an hour and a half when I noticed a small puddle of oil on the ground. I took the hoe off, but left the hoses together, to try to see where it was leaking from. I had the bucket curled all the way up and pushed the control valve to see if I could find the leak. Didn't find any leak when run against the relief valve a couple times. On the backhoe control valve, there is about a 3/8" hose coming from the valve that is open on the end and it just hangs down like a vent hose. It looks like this is where the oil was coming from. Can someone tell me what this hose is for? I'm wondering if it's for venting excess oil or pressure, kind of like a case drain hose on a motor but not plumbed back to the tank. Maybe it's supposed to vent some oil, like an additional safety mechanism? Of course I couldn't get it to leak much with the hoe not attached. The oil in tank was almost at the top of the sight glass when cold and above it when the oil was warm. I doubt it would be over full but I also thought the hose was so the reservoir wouldn't overfill when the oil got hot. If it's a wierd problem, I seem to get them. Any ideas why this hose was dripping and what the hose is actually for? Thanks. Dave

run the line back to the tank,or plumb it in a return line.it could be a kind of case drain,to let you know if a seal gave the ghost.Not very likely though.maybe in the past some one disconnected the line for that purpose.