I don't think such a port exists on my Grand L's. A picture of yours would be helpful.
I'm really not photogenic. Suffice to say the PTO back cover on both mine has a drilled and tapped port where you can attach a return hose. In fact, on my 540/1000 pto M9, there are two ports, one tapped and plugged, the other tapped and an elbow fitting installed. That is where the return from the loader goes back into the gearcase. The drilled and tapped port is on a centerline midway between the bottom of the PTO shaft and the top which dumps the return fluid back into the case at the correct level. My open station has just one. I added a Tee in both return lines to run a hydraulic motor on one of my implements. Was painless.
if you don't have a port in the PTO cover, Sure is easy to pull the cover and add a port, lots easier than what I see you folks doing. The back cover removes in maybe 30 minutes. Only thing you have to be aware of is the transfer shaft from the wet PTO clutch needs to stay in the clutch basket. Do not remove it. it has a loose coupler and if you remove the shaft when you pull the rear cover, the coupler will fall down into the gearcase, bad deal. It is 'fishable' from the fluid but getting the shaft back in the basket is a nightmare because the pto wet clutch is on the other side of the final drives and not very accessible. When you pull the back cover, the shaft won't come with the cover so don't pull it out. The PTO gearset and support castings are all part of the rear cover, so it is heavy and you'll need Kubota 3 Bond for reassembly. I've had both of mine apart in the past.
Easiest way to pull the rear cover without making a fluid mess is back the tractor up on ramps to tilt it forward after draining a couple gallons of fluid. No need to completely drain it.
I don't get all the complex plumbing, no need to in my view. Whatever blow your skirt up I guess. I like things simple.
Finally, you really need to give it a bath. Tractor is filthy. Better to see leaks when things are at least somewhat clean.
