I'm far from qualified to answer,but since nobody else has I'll jump in. 

First is the obvious.  Have enough hydraulic fluid in the rear end?  Yes, I know, it is almost an insult.  But I remember as a kid a neighbor whose motorcycle quit every week for a while. They'd load it up, take it to the shop, the mechanic would wash it, fill it with gas, and charge him $25.  Always check the obvious.

Does your power steering work right?  If so I would suspect a different problem.  I think those have a valve that provides pressure to the steering and brakes first and then to the remotes.  If that valve messes up the tractor will turn and stop, but nothing else works.

As far as where it is and checking it ... you are past my expertise.  I just  know it is there and when it is time to call my friend Stan the Service Manager.