Help Please - Leaky flapper valve Kohler 1-piece K-3402 toilet

Have you called Kohler customer service? What did they say?

One of the problems with many, not just Kohler, toilets is the corrosive effects of stuff that housewives toss in the tank to make the water blue or whatever. As the rubber damage occurs, small leaks expose some parts of the brass valve seat to more nasty stuff which erodes away that portion faster. Soon the valve seat is wavy, eccentric or anything but round and the rubber flapper will not seal. The solution to the problem is 1) replace the assembly (pricey) or 2) to clean up the brass seat area to match the rubber flapper. One commode in my fleet is a "Pompei" one piece (can't remember the K number) similar to yours. The valve seat corrosion problem arose nearly into it's second decade so I can't really complain. Ordered a bunch of repair parts, removed the assembly and serviced the valve seat area with valve grinding compound from an auto parts store. Any solid object about the size and shape of the rubber flapper can be used for lapping in the valve seat. A billiard ball is about the right size, but you may need several, it pays to be creative. In my case I have a buddy who owns a collection of neat machine tools and doesn't have enough projects to keep his toys busy so he tossed off a quick replica of the flapper ball in steel for me. And it works great now. The water bill went down $30 and there's peace in the valley. HTH

Joe