a serial number would help somewhat
but as steering systems go you'll need to check the pressure with a guage at the auxillary section of the control valve. This is where it gets it's oil from.
Sure the priority valve might be sticking but isolating the problem should be your main priority, the only way your going to achieve this is to check pressures at specific points-.
- control valve auxillary output
- orbital valve pressure out ports
- at the steering cylinder (pulling each line and checking for bypass flow)

You can start at the control valve then follow the circuit till you locate the pressure loss.
Steering pressure should be about 1400psi at the test port on the control valve. (since we have no serial number this is just an educated guess based on that class of truck)

you can also pull the priority valve out and take it apart and clean it... since the truck was sitting for an extended period of time it may just be stuck.