Brian
I would suspect the control valve, are the cylinder hoses quick couple at the valve? If they are you can switch the lift with the curl circuit and see if the problem stays the same. If it changes to the other circuit it is the control valve. KennyV.
Is it drifting down while in use or just leaking down when the tractor is off. Mine has always leaked down, but very slowly, when the tractor is off. It does not drift at all while running.
Welcome Brian, I have the same model and did all the fluids when I received
my tractor. There is also a screen filter/strainer to remove and clean if you are going to drain and refill. I ordered the maintenance manual from a local tractor dealer, pretty handy. There are quite a few drain plugs to remove to do a thorough draining, but make sure to be on a hard level surface to get an accurate reading on the trannny fluid level