I had rusted out tank to bowl screw so while apart I decided to install new flush valve. The rubber on it was in really bad shape. Got a new 2" flush valve. installed and put everything back together and had slow leak from tank to bowl. I lowered water level below the flapper and let it sit. The water drained out to bowl completely so the leak is not the flapper. Took apart and set tank on top of a bucket. put water in but not up to flapper. I could see it leaking under the nut on bottom of flush valve. So not good seal between the rubber cone washer and the tank hole. The hole is just slightly bigger than the washer. So cone washer barely goes into the hole. and the hole is a little rough. Ideally the hole should be about 1/8" larger in diameter so the washer seats further in but I don't know if anyway to do that. At least I would like to remove the roughness to try to get a seal. what is best way to do that? fine wet sandpaper?

 

A fine emery paper would remove the pits.
I always use a little bit of silicone grease on the washer. Helps it seal better.

 

did both of those, put back together and all seems fine. Thank you.