If the wings stay up when unhooked yes then it's on the tractor, but not in the tractor. It's the rear remote valve leaking by. They well leak down over time. Kubota remote valves aren't the best thing around, I think they need to hold a little better then they do. If that is what it is, a new valve might not fix it. Yes warranty should cover it. If you have more then one remote try putting into another set and see if it stays up better.
I don't know how you are going to unhook it with a load on it without a shut off valve. Be safe!!!!!