When there are two units using the same hydraulics, and only one pump, such as the PS, and the rest of the system, there is a valve to shift the fluid to the priority circuit when needed. This could be something like a shuttle valve. It might work like this. when you are steering, the fluid is shifted to the PS circuit, and the the cylinder operates. when you are through steering, the fluid is shifted to the normal tractor hydraulics. However, if the shuttle valve hangs up or is jammed in the PS mode, then you would not have any fluid in the other circuit, and all the fluid would go to the steering and back to tank.
Can you determine if there is a shuttle valve or divider valve, and determine if the fluid is being shifted when the PS mode is not being used?
The fluid from the pump has to be going somewhere, and if the PS is not activated, and none of the other hyd are working, then it would seem logical that the fluid is diverted from the normal flow, and and sent back to tank.
I think I would take the diverter/shuttle valve apart and clean or inspect it for any debris or galling.
If there was any blockage, then the relief valve might activate, and that is why I think the diverter/shuttle valve is not function properly.