It sounds like the orbital valve is going bad or the shaft seal leaking.
It's the hyd. valve that the steering wheel connects to that operates the steering.
I had one go bad on an old International tractor we owned some years back.
In my case, the best I remember, I had to pull the steering wheel, remove 2 hyd. lines, and couple of bolts that held the valve in place.
I sent it off to a hyd. rebuild shop and in a week or so got it back, reinstalled it and all was good.
You may want to look for other signs of leakage in the steering wheel area under the hood to be 100% sure.