My cistern inlet hole was seeping. I bought a new fill valve, removed the old fill valve, removed the sealant applied to cistern inlet hole, and installed the new fill valve.

What doesn't work

Tightening the plastic back nut (outside of cistern inlet hole) by hand doesn't compress the rubber washing seal (inside of cistern inlet hole) enough to stop the seeping. The seeping occurred at the same point as the old fill valve (outside the cistern inlet hole where the the back nut meets the cistern).

What does work

Overtightening the plastic back nut with spanner, i.e. 360° to 720° beyond what's recommended. I'm currently testing this using the back nut of the old fill valve. I'm also using a half inch plastic washer between the outside of the cistern inlet hole and the plastic back nut, like the old fill valve did. So far so good, but I'll wait a few more hours to confirm it absolutely prevents seepage.

The downside to this is that overtightening the plastic back nut on the brass shank of the fill valve could eventually overstress the nut to the extent that it cracks.

What might work

Using sealant. Will stop seepage and doesn't rely on risking overtightening plastic back nut.

Removing old sealant

I've gotten as much of the old sealant off as I can using a plastic card to avoid scraping cistern inlet hole. I think I need a sealant remover to remove what remains.


Cistern hole from above:


Cistern hole from below:


New fill valve:




Added plastic washer:


  • In the first fill valve picture of black washer and plastic nut, that looks like the washer only goes on one way. Are you putting it on the right way? Sealing washers work best if they are on smooth flat clean surfaces. Any defects to this and they might not seal as well.
    – crip659
    Oct 3, 2021 at 12:45
  • I put the washer on as pictured, i.e. domed face facing down. Maybe I've misinterpreted the diagram in the instructions (see third valve picture). In the diagram, the part of the washer that meets the cistern is diagonally sloping in, which I presume depicts the dome. I can face the washer the other way and test it. Oct 3, 2021 at 13:25
  • Personally I think the brass ones are better, the number of times that a plastic one has failed due to some idiot over tightning it and the plastic failing eventually creating a small leak is just crazy. A lot harder to have this issue with a brass one.
    – Joanna
    Apr 16 at 16:44

1 Answer 1


First off, and most important, get rid of all the sealant. You can carefully scrape it off with a razor blade or sand it off with some sand paper/emery paper. Make sure you have assembled the sealing washer correctly, with the small lip going into the hole of the cistern. I have always had to tighten the fill valves with a pair of Channellocs, same with PVC fittings that say hand tighten. Don't use that extra washer on the bottom of the cistern. It will work no but will probably compress further in the future and start to leak. Stay away from adding any sealant, it shouldn't be necessary.

  • Will the plastic back nut eventually crack if I overtighten it? To seal, I seem to have to tighten the nut with a wrench about 720° beyond what my hand's capable of. Oct 3, 2021 at 14:26
  • Should I be using a brass back nut instead of the plastic back nut? The fill valve shank is brass. Oct 3, 2021 at 14:35
  • 1
    @ClarusDignus I'd stay away from the brass nut. The PVC nuts have changed over time and I don't think they've updated the instructions. You see this all the time where instructions don't match the item being installed. My old valve had a larger nut with big tab on it, a big wing nut, and I still needed a wrench. just tighten enough to stop any leaking
    – JACK
    Oct 3, 2021 at 15:31
  • 1
    @ClarusDignus Again, make sure all the old sealant is removed from the outside bottom of the cistern as any that's left will make turning that nut much more difficult.
    – JACK
    Oct 3, 2021 at 15:47
  • 1
    @JimStewart I think, in most cases, any old sealant is due to people not knowing any better and not tightening up the nuts enough because they are afraid of cracking something. You've been around long enough so if you thought a coating of silicone grease was needed, it probably was and I think it would definitely work.
    – JACK
    Apr 17 at 0:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.