Hello JCLARK.
I have just finished reading through all 8 pages /forums/images/graemlins/shocked.gif of posts in this thread and there are a couple things I'd like to add to this discussion.
In your first? post you mention that you changed the oil with a low viscosity fluid and that is when you started having the problem. I know for a fact that a lower viscosity fluid will allow more internal pump leakage that will result in a lower system pressure. This slight difference in pressure could be the key difference in your being able to steer the tractor with a load in the bucket before changing the oil and not being able to after changing the oil. /forums/images/graemlins/crazy.gif
In a car engine using regular 10w30 motor oil, my oil pressure at idle was 45 psi cold. When I changed to synthetic oil, it lost 15 psi and went down to 30 psi /forums/images/graemlins/shocked.gif. My point is everything was the same except for the oil. The power steering system uses a lot more pressure than this and the difference between the type of oil that was in the tractor originally and what you put in there to replace it could be where your problem is.
And of course, this all depends on the 2 oils being of different weights to start with, but my money says that they are different. This would also explain why you noticed that the steering wheel would continue to turn when the wheels stopped. The steering wheel is attached to what is basically a hydraulic pump. Internal clearances in the pump are necessary in order for the pump to turn, but that same leakage will allow the pump shaft to turn when the wheels stop. There will be noticeably more leakage with thinner or lower viscosity oils than with thicker or higher viscosity oils.
Now, this being said, There are adjustable and nonadjustable pressure relief cartridges. If yours is adjustable, the dealer should be able to increase the pressure slightly and see if the problem goes away, on the other hand, you could put the original viscosity oil back in the tractor and see if it helps or solves the problem.
I have a Massey Ferguson 4225 utility tractor with 65 engine HP. The tractor has a 72" bucket and with AG tires on the front with 38 Lbs of air, I have very little problem steering the tractor on firm ground with a fully loaded bucket, however, if I drop the air pressure to 25 Lbs, the steering will not operate properly in soft ground. It will start to turn and then stop, to get it going again, I simply move the tractor a little bit and it steers no problem.
Hope this helps in some way, but if not, keep up the posts so we know what the end solution is. /forums/images/graemlins/cool.gif
Nailerguy