It crossed my mind but I didn't want to ask...don't you leave the PRV feeding at a preset pressure, as it is typically done. That way it's an auto fill system. I know some people don't leave it open for various reasons but that's another subject.
So it sounds like that control you have on the bottom allows you to shut it off completely.?? I wonder if that is just meant to be a normal feed or a fast fill and the normal feed is not working any longer. Pressure settings is typically done by adjusting that top screw but I'm not familiar with the various types that may have an easy adjustment with a knob.
I found that my PRV creeps up a bit over time, so I shut off the dedicated valve just before it. In my case I would crack the valve open once or twice a year. Not because it seemed low just to see if it went up much, if at all.
The bottom line is that unless there is a little leakage somewhere, you shouldn't have to bring up the pressure a bit, more often than about once a year.
Assuming also that you have a properly pressurized expansion tank and you're not losing anything out of the relief valve.
But for yours to drop all the way to zero seems strange, unless of course you haven't looked at in in a very long time.
