I went to check my home's water pressure. When I did this, it read off the charts at 100psi. Adjusting the pressure regulator did nothing apparent. But in the shower, the adjustment definitely had an affect - even thought he pressure meter didn't show a change. I tried a second pressure meter just to ensure it wasn't a bad meter.

I checked the pressure of my irrigation lines (before the home's pressure regulator) and the pressure coming straight from the city was at about 60psi. So, there was definitely something going on at or after the pressure regulator.

Finally, I thought to open a sink valve. When I did this, the pressure meter read the proper pressure. Adjusting the pressure regulator had the desired immediate affect. I adjusted the water pressure to 55psi.

But when I turn off the faucet, the pressure immediately went back up to 100psi. So, it appears the problem is the back pressure.

Is this still a pressure regulator problem. Will I need to replace it?

Or is there something else going on?

Do you have an expansion tank on your hot water heater?

I do not have an expansion tank. I am in SoCal so extreme weather is not an issue.

The purpose of an expansion tank is to "absorb" the extra pressure caused by the water heater heating cold water into hot water. As the water warms, it expands, thus increasing pressure. With a PRV, there is no place for the extra water to go. The air in the tank takes care of that situation.

Bear in mind this is an expansion tank. Don't mistake that for a pressure tank used on water wells.

An expansion tank is not the answer. Your pressure relief valve is obviously leaking across its inlet to its outlet. They are notorious for leaking. Some leak across the inlet and outlet from the day they were newly installed. Be aware, return the regulator, and get a new one that hopefully won't leak.