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I have a toilet that will start running as if the flapper is bad. (I’m confident it is not. I’ve replaced it twice.) It doesn’t drain much, just enough to trigger a fill every 20 - 30 minutes.

The strange thing is the tank only drains slightly and then stops. If I turn the water off it will only drain approximately 1/4 to 3/8’s of an inch and then stop draining completely. I can leave the water turned off for two weeks and it will never go below that level. I’m thinking if it were a bad seal it would drain completely after that time.

Is it a crack in the tank? I’ve just about had it and ready to replace the toilet. It’s approximately 20 years old.

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    have a good look at every component at the water level where the draining stops ..... there may be a small hole that allows the water to drain from the tank ...... when the water drops below the hole, then the draining stops
    – jsotola
    Commented Mar 8, 2019 at 4:36
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    @jimmy fix-it, make this comment an answer I have seen the siphon effect several times in the past the first time really had me scratching my head.
    – Ed Beal
    Commented Mar 8, 2019 at 16:13
  • @JimmyFix-it: As Ed said, make this an answer. Answers should never be relegated to comments.
    – R.. GitHub STOP HELPING ICE
    Commented Mar 8, 2019 at 17:20
  • As others have said, something is siphoning the water out. This is often because the tube into the overflow pipe is too long -- it should end above water level in the tank.
    – Hot Licks
    Commented Mar 8, 2019 at 17:23
  • @JimmyFix-it - I invite you to post your comment as an answer and then I will delete mine.
    – Hot Licks
    Commented Mar 8, 2019 at 22:01

5 Answers 5

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If the fill hose/tube (the hose/tube that runs from the fill valve to the overflow tube) is inserted into, rather than clipped above and aiming into, the overflow pipe, siphon action will lower the water level in the tank down to the distance the fill hose/tube is inserted.

Ensure the fill hose/tube is not inside of the overflow pipe below the normal water level.

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  • Yes! I have seen this exact problem.
    – Hot Licks
    Commented Mar 9, 2019 at 23:34
  • I will have to check this. That’s a good idea. Reading through the tips here (and with the water turned off) I filled the tank with a pitcher to the top of the overflow tube. In the last few days the water only dropped a fraction of an inch. Almost unmeasurable.
    – Razor
    Commented Mar 11, 2019 at 20:14
  • I cut about 2 inches off of the fill hose. So far so good! I think this was it!
    – Razor
    Commented Mar 12, 2019 at 12:32
  • So cool, I've never thought about that. Thinking back I have seen the refill tube actually inserted in the overflow a couple times. Great answer!
    – Joe Fala
    Commented Mar 12, 2019 at 13:55
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Maybe there is a small crack or leak near the top of the toilet tank overflow tube. See diagram below at red arrow.

enter image description here

(Picture Source: https://titanzplumbing.com/does-your-toilet-randomly-start-running/)

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Clearly the flapper - and just as importantly, the bottom of the tank where the flapper sits - is just fine. Otherwise you would have the toilet eventually empty when the incoming water is turned off.

You have some sort of problem with the fill valve and/or the overflow pipe. This (a) deliberately feeds some water normally into the toilet after the flapper has closed and (b) if the fill valve were to fail open (not a good thing as it would waste a lot of water very quickly), it would let the water drain down the sewer instead of overlowing the tank onto your bathroom floor.

A 20 year old toilet is old enough that the fill valve and other parts can start to fail. But new enough that standard replacements should work just fine (as opposed to 40 year-old 3.5 gallon toilets). There are plenty of choices (Fluidmaster, Danco, etc.) and the whole kit runs typically $10 to $25 - a lot less than a new toilet. I would avoid the old ball float styles - even if your existing toilet uses one there is no reason to stick with that - the new types are, in my opinion, much better.

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    In principle, the flapper could be damaged in a way that only makes it leak if there's high enough pressure above it, and the reduced depth after it's leaked a little means that the pressure is no longer high enough to cause further leaking. That seems very unlikely but it is physically possible, so your first paragraph isn't quite true. The asker says they've replaced the flapper twice, so I agree we can be confident that the flapper isn't the problem.
    – David Richerby
    Commented Mar 8, 2019 at 17:56
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    It is probably worth buying a kit which includes parts such as the bolts and bushings between the tank and bowl as well as the internal components. Take the old throne apart, give it a good cleaning, replace everything but the porcelain with new components and you'll have ruled out any broken components. I think most "kits" come with those but watch for ones which only include subassemblies.
    – Freiheit
    Commented Mar 8, 2019 at 19:56
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You probably have a crack in the overflow, the little tube that terminates just above the water line.

You may have to deform it a little by squeezing it for it to become apparent.

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It looks like Jimmy Fix-It’s suggestion solved the problem. Who ever installed the new float and filler years ago, did not trim the fill hose properly and it was creating a siphon effect.

Thank you Jimmy!

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