Tractor Repair: 970 case stops moving forward when steering wheel is turned., pressure relief valve, manual brakes

Question
Hi. I'm helping a friend work on his case 970. It has manual brakes and is a later serial number tractor. It originally had power brakes but he couldn't find parts to repair the power brake master cyl.'s so he converted it to mechanical brakes and installed the appropriate relief valve for the steering. This was done a couple years ago and all worked fine when he was finished. Now however he has a problem that makes the tractor stop moving forward, and possibly reverse when he turns the steering wheel. If he moves the steering back toward center the tractor moves forward again until he moves the steering wheel. We looked in the book and all we can find is that the steering pressure may be hitting relief pressure causing the powershift to kick out. the aftermarket pressure relief valve on this tractor is adjustable so we screwed the adjuster in which should have raised the relief pressure but had no change. I'm wondering if the steering pump could be bad and also be causing the powershift to go out of gear when the steering wheel is turned?
Thanks for any all help we get.
Allen.

Answer
  I think if there was a relief valve problem, the power steering would be weak.  It wouldn't cause the powershift to stop.  It sounds like there may be low flow from the pump.  The powershift and pto pressure is regulated at 195-210 psi by the pto control valve.  The steering pressure is about 1800-1900 and regulated by the relief valve.  If there is not enough flow from the pump, the steering will take too much oil away from the powershift.  Checking just the pressure does not accurately test a pump.  Pressure and flow have to be checked with a flowrater, or else the pump can be disassembled for inspection.  First make sure the filters are clean and the oil level is correct.