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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". All that being said, I probably could have just tightened the 3 brass post assembly onto the old o ring and it might have gotten me through another season, but why not replace as the old o-ring was 15+ years old and probably gonna need replacing soon anyway.

On my sprinkler backflow valve, 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". All that being said, I probably could have just tightened the 3 brass post assembly onto the old o ring and it might have gotten me through another season, but why not replace as the old o-ring was 15+ years old and probably gonna need replacing soon anyway.

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".

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On my sprinkler backflow valve, 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". All that being said, I probably could have just tightened the 3 brass post assembly onto the old o ring and it might have gotten me through another season, but why not replace as the old o-ring was 15+ years old and probably gonna need replacing soon anyway.