power steering pump problem

There were many times in my life that I didn't know the specifics of how a system worked, but I used the "replace every part till it works" method. This is much better when you have a supply of free used parts because otherwise it can get expensive, but regardless it generally works in the end.

In this case, you have replaced the pump TWICE, replaced the hoses and the belts and flushed the system, so using the "replace everything till it works system", logic would dictate that it HAS to be the steering box. Id go with everybody else's opinion on that too.