You could be experiencing thermal expansion. Do you have a thermal expansion tank on the cold side of your water heater?

Buy one of these Watts pressure gauges with the tattletale needle.

Do you have hard water? Did you see white deposits in the valve when you rebuilt it?

It could just be a bad vale.

We do not have a thermal expansion tank. We do have hard water, but I didn't notice any undue buildup when I disassembled the unit. One thing that just happened that might be relevant is that we got a lot of rusty grit the last time we opened the tub faucet a few minutes ago. (We do that so that any excess pressure on the house side is gone when we open the supply shutoff. That way we don't get the discharge or the drip).

I have shopped for the tattletale valve in the past but didn't find it readily. I'll try again.

Thank you.

If you have a closed system (and you do because you have this valve) and you don't have a thermal expansion device (expansion tank) then you are subject to dramatically high pressure spikes.

click here to learn about thermal expansion

That Watts pressure gauge is a part number IWTG and here's the link... http://www.watts.com/pages/_products_details.asp?pid=647 There's a where to buy button on that page.

Thanks- very helpful. Two locations within easy distance. I'll pick one up today.

Hooked up tattle-tale guage and haven't seen any dramatic spikes yet- seems to hover between 65 and 80 psi.

But wife may have fixed the problem by running the tub until she quit getting grit in the water. Since then it has settled down.

I called the backflow specialist at the water company and they acknowledged that the work being done on water mains a few blocks away no doubt caused water hammer and most likely dislodged some junk, and it is entirely possible for a backflow valve to be affected by it.

So I may have a solution; time will tell.

Thanks again for your help. I appreciate it a lot.

Leave the gauge installed for a few days and you may see thermal expansion spikes. If you do then install a thermal expansion tank on the cold side of the water heater.

I moved the tattletale gauge to an outdoor faucet which has several feet of exposed pvc. When the sun hit that corner the pressure went up to about 120 psi. I turned off the water to that pipe and drained it, so it currently is out of the system.

No more problems to date.