The problem started out when the tractor was used for a few hours,, the 3 point hitch arms would barely lift. Seemed like after the gear oil got hot. Then slowly got worse, 30 minutes of use, then after 10 minutes of use. So I changed out the gear oil, the 2 drain plugs on the bottom, and the ones on the rear axle hubs. I put 6 gallons of gear oil in, now the hydraulics will not lift the 3-pt hitch arms hardly at all. Do you have any ideas?