Power Steering issues after rebuild

System fluid pressure is controlled by a valve that screws through the reservoir into the pump where the high pressure hose attaches. Did the rebuilt pump include a new valve ? Sounds like the valve stuck causing excess pressure.
There's another flow controlling centering valve mounted on top of the gear box where hoses attach. I can't say what strange conditions that valve can cause.
I've seen first hand what a faulty valve in a steering rack can cause ( sudden hard left turn, or high pressure hose blow out ).