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The outside of my toilet cistern is dripping where the water inlet connects to the cistern and push valve, specifically, where the plastic nut (the one tightened by hand) meets the porcelain.

There appears to be a sealant applied to where the plastic nut meets the porcelain (see photos) and the drips appear to be coming from this sealant.

What do I need to replace to fix this? The valve, the washer, the nut, and/or the sealant?

The cistern is empty. The push valve is held by a string to prevent refilling. When I'm replacing any parts, I'll be sure to turn off the water supply.

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    I'm not an expert, but I don't believe the sealant should be there at all. That's probably the result of someone else bodging a repair job. Odds are good that the black washers inside & out will need to be replaced and the rest of it will be fine. It does appear that somebody used a wrench on that big plastic nut, which probably over tightened it, thus damaging the seals even more. They're made out of plastic with finger grips for a reason! It's possible that the wrenching may have damaged the plastic threads in the nut so you might have to replace the valve, too.
    – FreeMan
    Sep 28, 2021 at 12:05
  • Thanks for your insight. I'll get rid of the sealant, replace the washers and valve, and hand tighten the valve using my fingers rather than wrench. When you say black washers, does that mean plastic, rubber, or metal? Sep 28, 2021 at 14:24
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    I went ahead and made an answer out of my comment. I also addressed the additional question about washer material.
    – FreeMan
    Sep 28, 2021 at 14:39

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I'm not a plumbing expert, but I don't believe the sealant should be there at all. That's probably the result of someone else bodging a repair job. It appears that somebody used a wrench on that big plastic nut, which probably over tightened it, thus damaging the seals even more, leading to the application of some sort of sealant. That nut made out of plastic with finger grips for a reason - they don't want you to over tighten it! It's possible that the wrenching may have damaged the plastic threads in the nut or on the fill valve (tightening with a wrench could deform the plastic) so you might have to replace the valve, too. If, after reassembly, you've still got a leak, that could well be the culprit.

Odds are good that the black washers inside & out will need to be replaced and the rest of it will be fine. I would disassemble the valve to get to the washers and try to find replacements. They're probably rubber or possibly some sort of closed-cell foam. Whatever they're made out of, you'll want to replace them with new ones of the same material. I expect you should be able to find replacements at a local big-box DIY store. If not, I'd suggest trying an actual plumbing supply house. It's possible that you won't be able to find replacement washers and would have to replace the fill valve entirely, but that strikes me as an unlikely situation.

Before reassembling, make sure you carefully scrape all the sealant off the inside and outside of the tank/cistern. You don't want to scratch through the glazed coating or otherwise damage it. Make sure that any that's squeezed into the hole is also removed. If any is left behind, it will create a bump that even a brand new gasket might not be flexible enough to seal around.

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    Usually complete fill valves not that expensive and have all the parts that might need to be replaced. Check all parts before just buying the washers.
    – crip659
    Sep 28, 2021 at 14:47
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    A counterintuitive bit about rubber seal washers is that they don't seal when too loose, then they seal, then they don't seal again when too tight (whihc you are getting at here, not to say you were not.) An old credit-card or rewards card (plastic) makes a safe scraper for not damaging the ceramic when removing the sealant.
    – Ecnerwal
    Sep 28, 2021 at 15:24
  • @crip659 I'll bear this in mind. Do they typically have the finger grip plastic nut too? I've been eyeing a fill valve with a brass shank. Sep 28, 2021 at 15:53
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    @ClarusDignus if you go with a brass one (no reason not to), be sure to tighten according to the manufacturer specs. Most people's tendency is to think "there's water in that tank, and I don't want it on the bathroom floor, so I'm going to tighten it really tight to prevent it from leaking!" Unfortunately, tightening it "really tight" leads to cracking the tank. Thus the plastic bits that will deform instead of breaking the porcelain.
    – FreeMan
    Sep 28, 2021 at 15:56
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    @ClarusDignus probably time for a new question to ask why your new valve isn't sealing properly. Include one over view pic and some very detailed (and focused) pics of the washers and how they're sitting. That's probably the problem, or, perhaps, that it just isn't sitting properly
    – FreeMan
    Oct 2, 2021 at 23:58

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