i do not have any service documentation on this model but the program i use does have the parts breakdowns. Not going to be very helpful like a hydraulic schematic would be.
But from the symptom you describe and looking at the components in the steering system your problem can only be in a couple places.

- The steering wheel orbitrol valve (located at the base of the steering wheel shaft)
- The steering cylinder at the steer axle (located at the rear of the truck on the steer axle)
- the steering priority valve (mounted on the inside frame of the truck somewhere near the hydraulic tank)

I am assuming the location of the priority valve, you will just have to trace the steering lines to locate it.

Good luck, maybe someone has a 'hard copy' of a service manual and can send you the hydraulic diagram.