Assuming that the oil level is correct in the resevoir.... you've either got a steering cylinder that is bypassing internally.... or a bad steering motor.
I'd probably take one line or the other off the steering cylinder then turn the wheel the opposite direction and see if oil comes out the nipple on the cylinder end. If it does... then you have a bypassing cylinder that needs rebuilt. If no oil moves at all... you have a motor problem. That, IIRC, is a replacement issue at this point. No doubt you can rig something up that will work.