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I have a crack in one part of my sprinkler backflow preventer/vacuum breaker. It looks a lot like this:

enter image description here

Can I re-seal this crack? Or do I have to buy a whole new assembly?

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you should be able able to replace only the cracked part – ratchet freak May 20 '14 at 18:31
up vote 3 down vote accepted

It's a fairly standard quarter turn ball valve, for permanent repair, take the union loose, remove the valve and replace with like.

No patch you can apply will properly seal against expansion propagating through the patch material or breaking the patch material off eventually. Any moisture underneath the patch allowed to freeze will spall it off of the casting. Water pressure under the material tends to work it off akin to how the pressure in a blister separates the skin layers.

About the only thing I would recommend for patching is MarineTex which is a super duty industrial Marine Epoxy made for use on water containing castings in damp environments. You will have to sand the area to remove casting skin, extending the area beside and well beyond the crack, clean the surface thoroughly with MEK or Acetone and then mix up and apply the MarineTex per directions.

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Replaced it last night for $15. What made it really easy was having a quick-release on one of the pipes. – Dave Kennedy May 22 '14 at 18:15
Glad to hear, often the permanent fix ends up being the least expensive in the long run. Now be sure to drain everything for winterization, the crack probably was from a winter freeze. – Fiasco Labs May 24 '14 at 1:30

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