Facts: low pressure 20 psi. We had water logged pressure tank. Changed tank. Water has not gone into tank. Still only 20 psi. Did not install check valve. We have a submersible pump. If I turn off power to pump pressure holds at 20. If I leave power on but close the shut off valve after guage, pressure stays at 20. Do you have any ideas? Should I have put in a check valve? Thanks