My master head GM toilet acts like it is sucking air and losing it's prime when pumped. I have recently replaced the pump and stators. There is plenty of intake water but after 5-8 seconds of pumping the water starts to back up and fill the bowl. I have to plunge the toilet to force water into the pump to get it to drain. There is no blockage downstream of the pump as I put a guage on the line to check. Any ideas?