Priority valve, mounted on the engine driven hydraulic oil pump.
P/N D516514
It's being controlled by the LS line coming from the orbitrol.
Worth checking if it's not blocked between orbitrol and priority valve (could be a steel line-not sure, it's been awhile since i've worked on those)

The orbitrol acts like a small pump, therefore-with drive wheels off ground_ it will deliver enough oil to steer, could be the PSI that was measured.

Pump is easily tested; when the lifting works pump is fine, the priority valve could be stuck or not getting an LS signal from teh orbitrol.
Normally a visual check of the PV is done by lifting at a steady speed, when steering the lifting motion should be decreased during steering.

Good luck!