Power steering pump makes the "empty" sound all the time.

I had one on a Ford that blew the pressure hose in my driveway, and was shut off immediately. I replaced the hose, refilled the system, followed all the steps to purge the air, but it still whined like it was dry. After driving it for a few weeks it stopped whining, I guess it just took that long to get all the air out of the system.