water leaking into toilet even after changing flapper

"rusty twig" LOL

Avoid replacing the overflow; that can open a can of worms.

Check that water is not siphoning out the little refill tube that empties into the overflow. If the end of the tube is too far down the overflow this can happen.

Try shutting the supply valve and then add food coloring to the tank water. If you see color entering the bowl, try to stop the flow by pressing down on the (new) flapper.

The entire flush valve assembly (where the flapper sits)

Jim

I would change the overflow as a last resort. Not the hardest thing to do but you do need to take the tank off the bowl. If you do change the overflow make sure you replace the gasket between the tank and the bowl as well as the bolts that hold the tank on.

Have fun,

The rim where the flapper sits when closed can become rough, causing the flapper to leak. Feel it with your fingers. If it's rough, _gently_ smooth it with an abrasive. Start with something like a sink scrubber.

If it's chipped, or you scratch it badly, you'll have to replace the stand pipe. That requires removing the tank, so you might as well replace everything that isn't porcelain.

One possible cause is that the replacement flapper does not fit quite right. I had to exchange a flapper for one that cost a few pennies more just to get a proper fit even though they were nominally the same. It appears "they just don't make 'em like they used to".

Good Luck.

Also, make sure that the surface where the flapper rests is smooth. You might have some hard water buildup around the hole.

Try this: