Argthtjtr

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.

Maybe there is a small crack or leak near the top of the toilet tank overflow tube. See diagram below at red arrow.

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

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.

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.

Quoting from the comment by Jimmy Fix-it:

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.