Hey all, I'm wondering if there's an easy way to tell if my power steering pump has failed or if an internal seal in my steering box has blown out?

Quick context: XJ has been parked for 15 years, took it for a 250 mile trip over the weekend. PS fluid level was good when I left, and the power steering worked as normal for the first half of the trip. Second half of trip, fluid level was low (below the stick, but still some in the reservoir; topped it up) no power steering at all, doesn't matter if I'm idling or revving the engine at a standstill. Fluid level didnt drop for the second half of the trip.

Notes:
-I don't see any evidence of a fresh leak; although parts have been seeping forever I'm sure.
-Power steering pump makes the "steering lock" whining sound with any steering input
-The belt is tight and is not slipping

Do steering boxes have seals that could cause the pump to just "bypass" the power steering mechanism in the box? Is there a screen that could be clogged somewhere? Could the pump have failed and not be leaking/seizing? TIA