You need to know for sure if you have the single or duel stage clutch.
Can you push clutch about 1/2 way down and the forward motion of the tractor stops, but the pto continues to turn? If so, you have a duel stage clutch.
My 1957 to-35, I just drained my hydraulic/transmission sump and replaced the oil with gl-1.
When I have the lift in the up position, it holds the load up if I push the clutch in or turn off the motor. In a while the lift will go down, but I understand it was designed that way to take pressure off the lift system. My old a.c. tractor would hold the load up forever.
Good luck and let us know what you find wrong.