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We noticed water dripping down the wall below the toilet on the 2nd floor. I assumed it was a leak in the wax ring, especially because the toilet had become slightly loose and rocked slightly when sat on. So I pulled the toilet and replaced the wax ring and secured the toilet down tightly. It still leaked after being flushed. I then replaced the wax ring with a "Sani-seal" silicon toilet gasket. It still leaked after being flushed. So I tried reseating the toilet on the silicon gasket again, with the same result.

I then feared that the cast-iron closet bend had a crack or hole in it, because it does look corroded inside, and does have an indentation that I can see that could possibly let water through. I can't see or access the outside of the closet bend without tearing up the floor or the ceiling below it.

The curious thing is that when I pour a bucket of water straight down the pipe, I see no leakage. Similarly, when I reinstall the toilet and bucket-flush it (with a big bucket of water, pouring quickly), it still doesn't leak. It only leaks when I flush the toilet normally. It doesn't seem like it has anything to do with the supply line - just to be sure, I turned on the disconnected toilet supply line and filled up a bucket, and that did not produce a leak.

My question is, where would this leak be coming from if it only happens when I flush the toilet normally, but not when I bucket-flush it? Could it be that there is a hole in the closet bend that the water only reaches when it has the right type of momentum from being flushed?

Thanks for reading.

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I had a similar problem once but the leak was visible and the outside of the toilet bowl was wet. Flushing from the tank produced leakage but flushing with a bucket did not.

It sounds like clean flush water is somehow escaping on its way from the flush valve to the toilet bowl. This could be from a misaligned tank fitting or a crack in the ceramic material. I assume that you have found no water running down the outside of the fixture.

Depending on the design of the toilet, the connection between the tank and bowl parts may include a metal fitting set into the bowl. This can become loose or cracked and thus leak water outside the flush pipe but still inside the toilet.

Or an imperfection or crack in the ceramic could allow clean water to escape from the rim passage or wash passage, well, whatever it's called and the design of these passages varies a lot. The point is, flush water can escape its proper path and still not get outside the toilet where you would see it.

EDIT: couple of pictures to show what I mean:

image source: www.toilet-repairs.com image source: www.physics.hku.hk

The red callouts show where an invisible leak might occur only when tank flushing and not when bucket flushing.

  • I experienced this exact problem (leak due to bowl defect, as in your second image) on a brand new toilet. It does happen. – Jim Garrison May 8 '16 at 2:24
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As it turned out, there was no leak between the tank and the bowl, but it was a crack in the closet bend. When I looked into the closet bend, I could see what looked like a crack or pitting, but I didn't expect it to be the source of the leak, because it didn't leak when I poured water over it. Having exhausted other options, I decided to try fixing it anyway. I used a drill with a wire-brush attachment to remove the rust from inside the pipe around the crack, and then covered the crack with JB weld. I haven't seen the leak since. Sometime down the road I may have to tear up the floor or the ceiling to replace the closet bend because it is corroding. I'm still not sure why it was only leaking when I flushed from the tank and not when I bucket flushed, but it may have something to do with the momentum of the water as it flows past the crack.

  • It probably had to do with the amount of water in the flush. The tank can discharge up to six gallons per flush, depending on design and settings. Maybe your bucket wasn't that big. – A. I. Breveleri May 8 '16 at 5:14

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