I had the very same problem, a number of years ago. There is a pan, on the bottom, of the tractor, that has a huge screen, for where the pipe, for the hydraulic pump, pulls the hydraulic fluid from, that is probably plugged.
To access the filter, you need to pull the draw bar, and I think, there is 6, or 8 bolts, to remove, to be able to pull the filter plate.
You will either have to order, a new gasket, or you can get a tube, of sealant, called the Right Stuff, a little pricey, but worth the cost.
What I also did, was pull the top cap, flushed every thing good, with diesel fuel, installed a new return line filter, for the hydraulic system, and filled it to the fill line, of the dip stick, with trans/hydraulic fluid. Be sure, to drain the hydraulic oil, before you remove, the plate, for the filter. Hope this helps. Bruce.