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I noticed a small leak of water when I turned on the sprinkler system this year. The leak seems to be coming from the top of the pressure vacuum breaker. Note this is not the exact picture of what I am dealing with, its the closest I could find online:

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There are two screws at the top of this thing, should I be turning off the water and unscrewing these two screws...what could possibly be broken causing this leak? I really respect plumbers because boy do I hate leaks...

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If you turn off the water to your sprinkler system you can them remove those two screws and take off the top of the pressure vacuum breaker. Inside you should see an O-ring that makes a nice seal. Check that the O-ring is in good condition and then also look for things like spider webs or wasp nests that are preventing a good seal.

If water is flowing into the sprinkler system the O-ring of the pressure vacuum breaker will prevent the water from leaking out. If water flows the reverse direction, the pressure vacuum breaker will open and prevent the water from going back into your house (and you drinking it). So if something is blocking the O-ring from making a good seal, then you could end up with a leak.

  • The funny thing is I posted this at work and no one answered for a while...so I went home for the day. Popped it open and noticed the o ring rubber washer was worn out. Went to home depot and they didnt carry it, so I went to Ace / Aco (cant remember the name) and they had it. I replaced it and everything is fine now. I still will accept your answer because this is exactly what I did. – JonH Jun 15 '11 at 12:34
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    Ace has them. #39 O-Ring Bag. Part Number: 9D0035753B worked great for me. It was 59 cents. – user24921 Aug 31 '14 at 3:53
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On my sprinkler backflow valve (large black plastic cap with 3 screws), it was slowly leaking out from under the cap area. Turn off water. Remove cap via 3 screws. There is a brass opening with a needle valve through middle (and the 3 female posts the cap screwed onto). Turn the brass posts counterclockwise with pliers to remove. On mine, underneath, there was then a white/clear plastic ring (remove) then a needle valve with large red rubber plunger valve (attached to needle) - remove that. You will see a spring and red rubber lower valve (leave those). The leak is from the black outer (almost invisible) O-ring around the outer ledge of brass threads. Use a pin or something to pull up black o ring. Take to hardware and get exact size match to replace (new one may be more round, thats ok). Clean off surfaces of red rubber and white/clear ring with finger. Replace o ring pressing into channel, replace needle plunger valve, replace white/clear plastic ring on top of new black o-ring, replace brass center threaded needle valve holder/3 posts assembly. Tighten brass assembly with pliers down onto white/clear plastic which will push down & seal o-ring. Tighten firmly. replace cap with three screws. Done.. The o-ring size on mine was called a "#6 Union O-Ring".

  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know how best to contribute here. – Daniel Griscom Jul 3 at 21:37

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