Have you looked at the torsion bar between the rack and steering coloum. That tells the rack how fast to move left and right depending on how much turn force is applied. That is where the 2 power steering lines go through from the pump.
Or have you tried putting the old pump back in seeming as the problem started then. It also sounds like when the pump veins have been cooked. Raise the front of car and undo the tie rod ends and see if there is any binding when turning wheels by hand. Could have stiff ball joints. Hope this helps