Well, it seems to be fixed... for now, I suppose.
Recall, with all the faucets, etc., turned OFF, the water pump would continue to cycle. Certainly, its cycle speed was much slower than if a faucet was ON but the pump just would not shut off without flipping the pump switch to its OFF position.
First, I figured the Red Hat valve had worn out. Reading the Forum here I saw where Red Hat valves get old/weak and don't stop the water flow when in their OFF position. So, after nearly 20 years I simply decided to swap it out. The change was pretty straightforward with no changes required for the Pex plumbing. The difference is I replaced it with a "US Solid" ball valve at $50 from Amazon instead of a Red Hat solenoid "plunger" valve at near $150. The US Solid ball valve simply rotates 1/4 turn internally to shut OFF and ON the water flow, whereas the Red Hat valve has to use power continuously to hold the plunger to turn OFF the water flow. The ball valve uses only enough power to know it should stay OPEN and will not overheat due to its very low power usage. The Red Hat valve uses substantial power to hold the valve open. Red Hat states to not leave its valve OPEN under power for long periods because it will overheat. The US Solid valve has a little window with a red line that rotates with the internal ball valve so you can see at what position the ball valve is as it moves. And the US Solid ball valve only needs power to OPEN and stay OPEN. When the tank fill switch is OFF, the ball valve automatically closes. In fact, if the ball valve loses power for any reason, then it closes and stays closed until power resumes. Its default position is CLOSED.
But, alas, the pump still ran after changing the Red Hat out to the US Solid ball valve. Now what? I removed the pump and took it apart. There was a little dirt inside it but no particles of anything that would hold a seal open. As recommended, I tried holding back the water flow out of the pump with my fingers but couldn't stop it - too much pressure! I put my handy-dandy winterization hose on the OUTPUT side of the pump (rather than on the INPUT side when pumping in antifreeze), then kinked the hose to shut OFF the water flow. With the water flow completely stopped from coming out of the pump, the pump continued to run! Now what?
When I watched a video showing how to check out and repair my Shurflo water pump, it had a few words talking about adjusting the pressure switch using an Allen wrench on a screw at the top of the pump. I screwed it down a couple of turns and now the pump turns OFF when it gets up to its "shut-off pressure." Perhaps a temporary fix?
70F here today, no clouds, no wind, all doors on the house open, ceiling fan going to keep air moving. I love Yuma!!!