2000 Chevy Cavalier cooling system

I have a 2000 chevy cavalier with the 2200sfi motor, my problem started out as overheating. I changed the thermostat, flushed the system, the same thing happened. I then changed the water pump and inspected all hoses, heatercore and radiator for blockage and found nothing. I then replaced the head gasket, and inspected the head and what I could see of the block with the head off for cracks and found nothing that looked to be a problem. I reassembled it and took it for a ride sure enough it overheated agian with no leaks. After sitting a few days, I thought I may have purchased a faulty water pump so I got a new one and installed it and the seal blew instantly as soon as the system was full of water. Again I got a new pump and the same thing happened to it. The coolant is circulating now but the pump leaks bad, my question is what can cause a water pumps seal to blow instantly like that? It does not seem to have any blockage in the system?