Don't know much about troubleshooting steering problems, but I'm curious as I have a 665. The steering pump shares the same reservoir with the transmission. The easy stuff would be ensuring plenty of fluid, filter not blocked. What it's doing? Can you describe the symptoms? If you jack the front-end up, do you get any wheel movement. Likely, the problem would be in the steering block, the pump or the cylinder. I have the service manual for the tractor and can post up some pics of the steering section if you need them.