Hi,I installed a 1/2 hp pump,to pump water out of a canal.The intake sets only 6 ft. below the pump.When I turn on the pump,it works fine, until it runs out of water. Then the pump keeps running.It has a 30 / 50 pre-set pressure switch.It has a built in check valve.No pressure tank.What do I do to get the pump to shut off when it runs out of water? It's a new pump.