Would be good if you could get the hyd / trans oil hot then drain system.
replace oil and filter
If this does not correct the problem, then you will have new oil if you must have the system repaired.
Have you drained water off the bottom drain????
Tractor needs to set for a few days before you attempt to drain off water.
Be sure to buy the correct hyd oil from dealership.
do not use cheap off the shelf oil