I have removed working power steering pumps..vane type..left them on the bench for a couple of weeks while working on other thngs, and voila..when put back in the car, no power steering..had no problems pryor to removing it from the car.

Removed it again and opened it....the old oil had turned to a varnish and basically locked up the vanes in the rotor while sitting on the bench drained...looks like the air reacted with the ancient steering fluid and turned it into a varnish....Pulled them all out, cleaned thoroughly and replaced everything...pump re-installed and works like new....

How many of us ever think to drain and replace power steering fluid...mostly just add as needed right...