MYSTERY PART - and it's LEAKING - please tell me what this is

[amateur_plumber]

In your picture , just to the left of the prv you replaced I think I see a small pipe sticking up that is threaded into the tank tee . Is there a small box with wires going to it on top of that small pipe ? If so that would be your pressure switch . Did you check the air pressure in the pressure tank while you had everything drained ? If you have a submersible pump ( a pump that's down in the well ) the pressure switch is usually on or near the tank tee . If you have a pump you can see in your basement the switch might be mounted on the pump .

Yup - there's a box with a "Square D" logo on it, and the electric runs in and out of there. I assumed it was just boosting the voltage up to 240. When I get home from work, I'll pull off the cover (power off first) and see if there's any obvious rust or corrosion.

Forgot to check the pressure on the drained unit - I'll go to the other 'sticky' thread and run through the maintenance procedures that the moderator posted there.

[WV Hillbilly]

That's your pressure switch . When you remove the cover you'll be able to see a set of contact points . When the points are closed ( together ) the pump will be running . When the pump kicks off the points will be open . These points usually arc as they open & close & over a period of time will become burned or pitted . These points are what you need to check when you look in the switch . Also the small pipe the switch is mounted on sometimes clogs up & causes erratic operation of the pressure switch . If you end up taking the switch off use a screwdriver or something & run it down inside the small pipe & work it around a little bit to make sure the pipe is clear . Don't forget to turn off the power before checking the switch .