Start taking off hoses. First at the cylinders in the back, start the machine and turn the wheel left and right (a pail under the hoses might be a good plan!). Oil there = good, just a problem with the cylinder (Seals). Oil not there go further...
Do any of your other hydraulics work on the combine? Coming from the same block (according to the case online parts) are the header lift, unload auger swing, and reel swing circuits. If none of these work its something to do with getting oil to that control valve or the oil is not getting through it. If they all work...
Also, on the steering return line is the inching pedal. If that works but no the steering then its something in the hand pump.