Power Steering Power Shaft Sheered in half

I have 1996 lumina. The power steering quit working. I replaced the pump. The old pump had sheared the shaft into. I had never seen that before. I replaced the pump with a remanufactured pump. In a week I had the same problem. It sheared the shaft into again. Any ideas what would be causing the shaft to break?
Thanks,

I don't know but any time you have a catastrophic failure of a pump, you'd want to flush the entire system before putting a new pump on...because whatever the original pump "ate" to jam it up is still in there, plus now shotgunned with debris from the pump failure...your steering rack will be next...