My understanding of those systems is that Ford for example, of that era, used a "power assisted" system that put out around 600 PSI. The system would steer manually but if the assist was functioning the steering force was greatly reduced.
I just acquired a Case-IH 895 that had the same problem, using a hydraulic cylinder up along the front axle to do the work in a total hydraulic capacity, no assist like the Ford. He said that it wouldn't turn while sitting but if rolling it would steer OK.
I told him to keep the loader and bought the tractor. I had no problems steering without the weight of the loader........soooo my best guess is that either the 1500 or so hydraulic pressure PSI, if i recall my service manual correctly, isn't 1500 PSI or the cylinder up front has a bad case of leaking seals letting the pressure bypass the cylinder rather than push the cylinder and perform work.
Since mine is working fine without the loader installed, I passed on a cure. However I did contemplate just how I would rectify the problem and my thrust was to decide if it was low pressure which would indicate a problem in the steering wheel shaft assembly, or the fact that the pressure was there but the cylinder's seals were leaking.
Moving right along, my first inspection would be to disconnect a hydraulic line to the cylinder and install a pressure gauge thus blocking the flow. I would then turn the steering wheel in the direction that would force pressure to that hydraulic line and see if the pressure indicated met the requirements in the service manual. If it did, then I would get on ebay and buy a new cylinder which was very reasonably priced to my amazement.
HTH
