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Discuss 1/2" bottom entry fill valve not sealing well in the Fittings & Pipes area at

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  1. Royston Shufflebotham

    Royston Shufflebotham New Member

    Hi all,

    I fitted a Fluidmaster 1/2" bottom entry fill valve in a toilet about 4 years ago, and it's all been problem-free until last week, when the seal against the cistern began to leak.

    Looking at again, I'm not entirely convinced about the sizes of the fittings:
    the water pipe fitting underneath the cistern is 1/2", the hole in the bottom cistern is 3/4". Looking at how the valve mounts to the base of the cistern, there's a rubber seal on the inside, but I'm not convinced that it's sitting well in the 3/4" hole. On the outside, there was only a wimpy plastic nut which barely covers the 3/4", hole with no seal that side.

    I've tried refitting it, and adding a 3/4" poly washer (from a previous siphon-repair adventure) on the outside, but it's still not sealing that well.

    So, some hopefully quick questions for you experts:
    1. Am I using an appropriate valve?
    2. Is it right that there's only one rubber seal on one side of the cistern mounting?
    3. Can you recommend anything to clean around inside of the cistern around the hole before refitting a seal? There's a small amount of limescale there which is hard to get to, and may well causing sealing issues. I'm resisting using citric acid or Viakal or equivalent to dissolve the limescale in case traces are left behind and attack the rubber seal.
  2. rpm

    rpm Trusted Plumber Top Contributor!!

    1 Yes
    2 Yes
    3 Scraper
    • Like Like x 2
  3. ShaunCorbs

    ShaunCorbs S. Mod Trusted Plumber GSR Top Contributor!!

    As above and sometimes you can use a tap back nut they have a bigger flange area
    • Like Like x 2
  4. Royston Shufflebotham

    Royston Shufflebotham New Member

    Thanks very much for the quick replies. Really helpful to know I've not got it totally wrong. Yep, flanged backnut looks like a much better idea for the outside.
  5. rpm

    rpm Trusted Plumber Top Contributor!!

    Don`t Gorilla tighten it!
    • Agree Agree x 2
  6. Best

    Best Trusted Plumber Top Contributor!!

    Or you could use a ballvalve back nut - type that has a shoulder to centre the valve in the cistern hole.
    Need to check though that the nuts shoulder does fit the hole.
    • Agree Agree x 1
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