Hi. I'm having difficulty reaching 50psi in my water pump. It reaches 40psi and just continues to run. I've adjusted the air pressure in the pressure tank, but that didn't work. The tank had no air pressure when I checked it earlier today. I followed the procedure listed on the air tank to get accurate pressure (drained down the water pressure and rebuild). What could be the problem? The pump is 3 years old and I've never had this problem before.