I have a toilet tank that's slowly leaking into the bowl causing it to refill periodically. I can rule out the fill valve having loose grit/obstructions causing it to fill slowly after it should have shut off.

The leak has to be one or more of the following:

  • flapper
  • bolts and corresponding rubber gaskets
  • flush valve gasket/nut

Here's a photograph of the tank:

Toilet Tank from above: fill and flush valve

I don't know if the flapper is leaking or one of the bolts or flush valve gasket. I was pretty sure that I had a waterproof seal on the bolts and flush valve because before I remounted the tank to the bowl I filled the tank with enough water to see if it any of those were leaking. (It leaked a bit at the flush nut before I carefully tightened the flush valve nut a quarter turn beyond hand-tight).

If the problem is with the flapper I probably won't need to pull the tank back off to fix. Otherwise I'm doomed to that again.

I've thought that if I tested the tank by filling with just enough water to cover the bolt heads/portion of flush valve below the flapper, I could see if it leaked. If not, then it would probably be the flapper.

Any other idea to figure out where the leak is without pulling the tank? Putting a dye in the tank won't help because the two bolt connections as well as the flush valve all drain into the bowl flange, so I'd see the leak as color in the bowl in any case.

EDIT: I wanted to add some information and correct some of my misunderstandings in the question I'd posted. Also, I think I have the fix for my case and will try to post a separate solution.

I think the toilet bowl I have is pretty typical. Here's what the tank connects to:

enter image description here

My belief that a leak in either bolt connection would leak into the larger hole and end up in the bowl is incorrect, especially with the gasket covering the flush valve nut. It is the case that correcting a leak from the bolts or the flush valve hole involves removing the tank.

I thought it was being smart when I had tested the tank with its new valves for leaks before reinstalling it on the bowl. What I didn't realize is that I did it wrong. The amount of water I used for the test (just covering the bolt holes) didn't cause enough pressure to test that the flush valve didn't leak. Later, testing with water up to the water line in the tank caused significant leakage around the flush valve.

An additional note is that I'd done a recent overhaul of the tank, both the fill and flush valves (using a kit containing both parts). So the tightening of the flush valve nut was something I'd done recently and not from the previous setup.

  • Additional information should be edited into the question field and not in the answer field. (that info would have informed my answer). If you fixed the problem then you can post an answer in the answer field. Basically you are saying that you installed a new flush valve (new info) and did not tighten the nut enough and water was leaking on the inside of the donut and into the bowl?
    – Alaska Man
    Commented Sep 9, 2020 at 16:51
  • I have a suggestion: why don't we just delete the post to begin with? Maybe someone with sufficient privilege reading this can do that. It could replaced by someone else with your question and answer above or none at all.
    – gcbound
    Commented Sep 10, 2020 at 13:07

2 Answers 2


If the leak is only from the tank to the bowl, and not on the floor, then the only option of the three you listed that can be the cause is the flapper.

Judging from the photo your flapper can be replaced without any disassembling of the tank.

There are two flexible tabs on the back of the flapper that go over two fixed pins on the bottom or the overflow pipe. You can reach down and pull the tabs off of the pins, and reverse the process to install the new one. Very easy.

There are two size's of flappers, 2" and 3". You can shut of the water supply valve and remove your old one and take it to the store to match if you are unsure of the correct size.

  • I don't know the terminology, but the toilet bowl had a "drip pan" shape at the top of the bowl that covers the bolts as well as as the flush pipe nut. So to tighten the bolt nuts or the flush valve nut, for example, or to check for leaks, you have to remove the tank from the bowl portion. In the image, right above the seat you can see the start of where the bowl is flanging outward to cover the nuts.
    – gcbound
    Commented Sep 7, 2020 at 21:42

What seems to have fixed the slow leak into the bowl is painfully simple:

Tighten the darn nut, but don't overtighten the darn nut.

Here's a picture of the bottom of the tank with the flush valve installed:

View of tank bottom showing flush valve nut

The instructions that came with the flush kit I bought suggest you tighten the flush valve nut (only) hand tight and warn about tank cracks if overtightened:

enter image description here

I put a tiny tick mark on the bowl to indicate where "hand tight" was at. In my original test of the tank I'd only filled the tank an inch or so with water, and my hand tight valve was waterproof. As it turns out later, adding water to the normal fill line caused a large amount of water to leak.

What ended up working for me was to use adjustable slip lock nut wrench shown in the image above. Alternative tools for tightening the nut, such as channel locks, would tighten it as well. In order to stop the leaking flush valve, I had to turn the plastic nut carefully past hand tight to the small tick on the nut (almost a half turn). The danger is to torque the wrench too much and to crack the porcelain of the tank. It's something I'm sure a plumber would master, but if you're a newbie like me, getting just the right tightness is tricky sometimes.

Anyway, that appears to have stopped the slow leak. I'd still wanted to test the tank before installing it on the bowl, so I put it up on a few scrap 2x4s and filled the water up to the normal water line. No drips:

tank on wooden supports filled to line and showing no leaks

I know the whole process shouldn't have been so difficult, but I thought I'd done it right the first time and as it turns out I was too wimpy! Hope this helps someone.

  • I am glad you got this fixed but really no one should get the idea that they need a adjustable slip lock nut wrench to install a toilet. I have also never heard of "hand tightening" the nut on the tank. I also HIGHLY doubt that going past a half turn further than hand tightening produces a risk of cracking the tank. Especially with a plastic nut. Again glad you got this fixed but this is simply a confusing answer as it preys on myths that people shouldn't really know about.
    – DMoore
    Commented Sep 9, 2020 at 3:47
  • I'm sure someone could use channel locks or a cheaper tool. I used the wrench because I had it from past faucet/sink work. If you have one, or a neighbor is selling an adjustable wrench for a buck at a garage sale, I think it'd be a great deal for toilet work.
    – gcbound
    Commented Sep 9, 2020 at 4:21
  • Searching online for something like "tighten a toilet flush valve nut cracking porcelain" will give you a number of articles mentioning the possibility of hairline and worse cracks in the porcelain from overtightening. I really want to believe that those are myths and my own experience worked fine.
    – gcbound
    Commented Sep 9, 2020 at 4:31
  • Show me where it says nothing beyond a half turn past hand tightening....
    – DMoore
    Commented Sep 9, 2020 at 6:59
  • 1
    Unfortunately, "hand tight" is a vague term, at best, based on whose hands we're talking about. The implication is to not use tools to keep tightening until the tank cracks because watertight will be well before that. One should be able to hand tighten until watertight, but not everyone can. Using some sort of pliers to gain leverage to tighten is a great idea, so long as it's done carefully. I also like your test rig.
    – FreeMan
    Commented Sep 9, 2020 at 14:58

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