Power steering and rack help
 

I've been having an on going issue with my power steering that started about one year ago. I have a 2000 ranger automatic with a 4.0l and 231,000 miles. I originally replaced the power steering pump and a few months later decided to replace the entire rack. When I replaced the rack I also replaced the power steering pump so that I had an entire new system. Three months later the power steering pump started losing fluid and the bearing failed. I replaced that pump and the same thing happened. Since then I just let the pump run dry until it just failed. I'm not sure what is going on. I thought I had read that there is a check valve on the rack. Thanks for any help!