I have a Goulds pump, model VJ10, 1hp, mounted on to a 2" packer well adapter. The pump includes a model AV21 pressure control valve. I cannot find any documentation as to the depth of the well but I have spoken to neighbors and all say thier wells are over 100' deep. The well itself is over 20 yrs. old and I have put several pumps and tanks on it over the years. My problem is the pump pressure will drop even with the main valve to the house closed. I have installed shut-off valves between the house and the pressure line and also between the pressure line and the air bladder tank via a t-fitting in the pressure line to try and determine the problem. With the main line to the house closed, if I try to close the air tank water valve while the system is under pressure the pump begins to cycle very rapidly. My question is does the AV21 pressure control valve on the discharge side of the pump keep the pressure in the line or is this a problem down in the well itself ? Can I install a one-way valve between the AV21 and the pump or just replace the AV21 or do I need to pull up the well pipe and "Packer" in the well ? One more piece of info, the pressure hose for the pump Elec. switch appears to come off of the AV21 pressure control valve on the "House" side of the valve if this makes any differance. Thank you for your time.