Rereading the posts I see that you said the hydraulic fluid level was ok earlier, but then you said that you see no fluid on the marks in the sight glass at the rear of the tractor irrespective of how it is parked. So what causes you to say the hydraulic fluid level is ok? Are you checking the power steering fluid tank that is under the left step? That is separate from the rest of the hydraulic system.
You must have some fluids or the rear hydraulics and 3 pt hitch would not work. Again, as others have said we are talking about a combination hydraulic and transaxle fluid that resides in the transmission and differential area. If you never have changed the fluid in this area, then you need to do so. That is the only surefire way I know to verify how much fluid you have. And your hours would require a change. If you have changed your transaxle fluid then please disregard this and the next paragraph
Get the hydraulic filter and about 10 gallons of premium hydraulic/ transaxle fluid. I think it uses 8 + gallons, but I need to go back and check my manual. I use the Tractor Supply premium fluid and have had good success. It is best to use the filter from the dealer imho. Park the tractor on level and drain the fluid and refill until you you see the fluid level appear in the rear sight glass between the two lines. Please let us know how this goes. W Jones