No Power Steering after replacement

I hope you can help me, I am a mechanic, working on a vehicle in my garage. It came to me with no P/S, the pump was making noise, and could see not moving fluid inside of pump. I pulled the belt to check for pump shaft end play, and the shaft came right out with the pulley. "Bad pump", I said, so I replaced it with a new one. Put it back together, after flushing the system, although not real well, still no P/S. So, I got a used steering box from a reputable supplier, and installed it, still no P/S. I removed pressure line at steering box, and it pumps very hard and fast, but I have no gauge to check PSI with. Remove return line from steering box, and it comes out slowly, but seems ok. The system has a cooler, but even with return off of box, there is no P/S, so it can't be a restriction there or anywhere else. The only part of the system remaining is the pressure switch, but I don't see it being the cause. The only thing I can think of is that I got a bad used box. I have asked multiple other tech, and they offer no help.