Checked a couple information videos online reference the plastic Anderson valves. The information on these said you need to shut the city water off to relieve pressure when switching from one setting to the next or you will blow the 0 rings out. I have never done this with any of my 5 RVs over the years. Are they making them these valves that cheap now?
One should ALWAYS relieve the water pressure before turning the valve...this includes your comment of killing city water and also includes shutting water pump off if boondocking. Neither of these will help much until you **open a faucet and relieve all pressure on the coach side also.**
I've replace our Anderson cartridge once 3 years ago with a new one. The old one had a ripped O-ring as I suspect yours does. First ascertain it is indeed that valve and not your water pump check valve. If the Anderson, your choice are:
1) Pull the cartridge and verify a bad O-ring.... source out a replacement one.
2)Attempt to locate a dealer that MAY have an old OEM cartridge in stock as ABCO has reportedly quite manufacturing them in favor of their newer brass valves.
3) Buy a new brass valve as I did.
They state it is a 6 week wait, and ours took 5 weeks to ship with another week to arrive to my niece's in Oregon. Coordinating a shipment depot would be near impossible as we move around so often...we don't know where our next stop is, only where we've been, kind of boondocking...lol. We were 4 months away from our summer long-stay area in Oregon, so that was the safe place to ship to without it getting "lost in the mail".
We are FT boondockers and a couple months ago, I filled our tank, then turned said valve to pressurize the trailer when done....pump just cavitated. Tried to remove the cartridge with no luck and was afraid if I did get it out, I would not be able to replace back so to use our pump to continue our 4 month journey back to Oregon.
Long story, but I still have to swap a couple hoses to refill our fresh tank, then swap them differently to have pump draw from tank in order to bypass the defunct Anderson valve.
Good luck with your endeavors.