Wow! Gone a few days and have to read a book to catch up. To summerize, replaced tank, which I agree was bad, but did not fix problem. Second check at tank would probably stop water hammer on pump shut down but, hydrant would not work, and can cause a vacuum when pump is off. Second check valves usually cause water hammer on pump start up, not pump shut down. Conclusion; Check valve at pump is probably bad, replacing should fix the problem. The draw back is that you have to pull the pump. By the way, what caused the check valve to fail in the first place? Even with a good spring in a good check valve, the valve will only be open according to how much flow is moving through it. The closer the check valve is to the closed position when the pump shuts off, the less water hammer happens. Restricting the flow before the pressure tank with a valve, puts higher back pressure on the pump. Higher back pressure on the pump does not hurt anything. When the pump sees higher back pressure it just thinks it is in a deeper well. Your pipe just has to be strong enough to handle the pressure, the same as it would for a pump in a deeper well. Just check your pumps max pressure to see if your pipe is OK. As the tank is filling at 1 GPM through a CSV, the flow through the check valve is also 1 GPM. At 1 GPM the check valve is only open the width of a piece of paper, regardless of spring in the check being good or bad. When the pressure reaches 60 and the pump shuts off, you will not hear the check valve close or any subsequent water hammer. I suggest installing a CSV before the yard hydrant. If the CSV does not fix the problem, return it for a full refund. However, if the CSV does not stop the water hammer, then your bottom check is really, really bad. The check would have to have loose, broken, or missing parts. Remember when I ask what causes this problem? Like most other items in a pump system, the check valve and the bladder in the tank, where destroyed by repetitive cycling on and off of the pump. This causes the check valve to basically slam itself to death, and the tank bladder to break like repeatedly bending a wire. If it is already too late for the CSV to salvage the last remaining life left in the check valve, it will certainly keep this problem from happening again to a new check valve. The CSV will fix the root of the problem which is cycling. Anything else, including just replacing the check valve, is just a band aid.