It can be serviced but before you go off calling a plumber give something a try. Now I don't know what model you have so I am just giving a broad idea of what to do.
On the top of the valve there should be 3-4 bolts or screws that can be taken out. Shut the water off on both sides and take the top off of the valve. There should be a whole cartridge that comes out along with some seals. Just take that out and rinse it off real well. A lot of times you will get some dirt on the seals and it won't allow it to close all the way. Once you rinse it off you can put it all back together and turn the water back on.
We have our valves at work leak at least once a year and 90% of the time the above fixes it.
If that still doesn't do it you can search for a brand new cartridge or call the dreaded plumber.
Hope that helps. Feel free to ask any more questions.
Matt

Thanks for your help, Matt- you got me started on the right track.

It's a 900 M3, which, it turns out, is subtly different from the M2 and seriously different from the original M. (The original M had four screws, but the later versions have a "straddle" with two screws. M2 has a different internal part than M3 but is indistinguishable from the outside. Keep your ID plates clean!) But repair kits exist for all of them, and even though the kits ain't cheap they are a LOT cheaper than a whole new valve. Thanks to you, I can probably do without either, and my worse-case now is putting up with the drip until I can locate the parts, assuming the simple remove-clean-replace option doesn't take care of it.

I found a very helpful link which you might be interested in.

Thanks very much.

Here is a link that might be useful: backflow devices info

900? no such thing, you mean 009, 909 or 9D?

Glad I could help and I hope the cleaning it out does the trick for you. We just had two of our three fail inspection last month. One was able to have a rebuilt kit installed for about 350. The other needed a whole new install as it is obsolete. Only cost the company about 3 grand. And I don't even want to see the bill if our twin 8" backflow goes!
Good Luck!!!
Matt

zl700- You're right- boy, did my dyslexia kick in or what? Thanks for the catch.

Matt- we had to leave town yesterday and "solved" our problem with a 5 gallon bucket for the night. Now this morning it has quit leaking. So I guess it has a good chance of being an intermittent type thing even with a new repair kit. I'll keep that bucket where it is.

Thanks again,

Mark

It's a week later, the repair kit is still on order, and the valve is still dry. I'll leave the valve alone, but add a sump pump, which I need anyway, to my setup.

Thanks for all your help,

Mark

I have a Watts 009M2 1 inch valve connected to my underground sprinkler system. It was leaking and I removed it and started looking at the inside. The pepair kits are expensive and the Watts dealer in the area will not sell me anything. They said they don't sell to the public. I decided to replace the whole valve instead. I was surprised to find the same model Watts 009 M2 one inch being sold by different web sites all the way from $172 to more than $900. Am I missing something here?
I ordered one for $172 plus $12 shipping. When it arrives I will simply install it and hope it will last 12 years like the old one did. Any comments and advice will be appreciated.

Dave

did the $900 come with a plumber for installation?