i had this problem, stripped both pumps down, cleaned them and re assembled, still no joy, are you winding hose up with tap valve open & empting the hose of water

, this was my problem, as when you turn the valve off air is trapped inside the hose, when turning pump back on the water is pumped through the empty hose, forcing the trapped air inside hose backwards into the pump and back into tank,i started winding hoses up with valve off, keeping water in the hoses, problem solved

Brian.