If you only had a problem with the pump leaking, and the steering wasn't erratic prior to the pump replacement, the liklihood of having a bad rack is really really really small. When we replace a rack or a pump, after flushing the entire system, we head to the dead end street just two blocks down and circle to the left until we are dizzy, and then circle to the right until we are dizzy, repeatedly. You would be amazed at just how long it takes to work out insidious air bubbles. If, after driving for awhile, the problem doesn't rectify itself, I would probably double check the belt. Since you changed it as well, did you use the same size belt? and did you route it the same way? It really sounds like air, or a slipping belt to me.

Hope this helps,

Chris