The oils comes from the main hydraulic sump. There is two hydraulic pumps on the tractor, one is for the implements (3PH, FEL, ect,) and the other is dedicated to the steering. The only filter is the main one on the transmission.
It's possible that the pump is heading south, or there is a kinked hose to the cylinder, or the steering valve is going bad. You will probably need the service manual to see how to check the flow and pressure of the pump.