I have a starange one here. I installed a new bladder type pressur tank on an existing shallow well pump. Thge old pump was cycling very frquently and the old tank was very water logged. The pump is a 1/2 hp with a pressure qauge and the cut-in /out switch mounted directly to it. I installed a check valve in between the pump and tank and a pressure gauge on the T from the tank. I did nothing to the pump except switch out the pipe nipple going to the pressur tank. The switch was originally set at 20/40 and when I plugged in the pump it would go to psi at the pump and about 43 psi at the tank but would not shut off. It would flutter at 50 psi and try to shut off but would not. I unplugged the pump to avoid burning it out. I drained the pressure tank and reset the psi to 18 from the original 28 psi set at the factory. I plugged in the pump and again it would not shut off. A new 30/50 pressure switch made no difference. The pump ran great before the tank switch and yet I did nothing to the pump except the nipple change. Any suggestions? By the way this is a great site for frustrated home plumbers !!!