Kohler Toilet Replacement Parts

Recently started leaking from the tank to the bowl (like a slow leak). The valve is new and the drain, it is supposed to seal is clean (sanded smooth w / 600 grit sandpaper). He fled permanently and therefore never empty (waste water). The other thing I learned is that Kohler, although supposed to be good, do not have the quality replacement parts available (went to three locations including hardware and plumbing stores). Also available online, but no way to determine which parts numbers are correct / apply. All manufacturers of other toilets are many parts, including off the mark. So after all that walking, here is the question. Can there be another entry point other than the valve leak? I do not see the world, even the pipe valve valve hooks seems tight. If I hold the valve by tightening again I can not completely stop the leak (even if it is small). Are there any typical problems like Kohler along this line? Looks like a pretty easy problem to fix, but does not turn to be. Any help would be appreciated. Thank you.

Flush the toilet when he reloads, cut off the water when there are about 2 inches in the tank. Open the tank, put in one or two drops of food coloring. Watch the flow of food coloring, it will lead you to where the leak is.

Still probably the valve. Even the new can be bad.

If your toilet fill cycles, that is, it fills, then stops, then meet again after a few minutes, then it seems that the valve seat may be damaged. Empty the tank and remove it. Replace the seat and the valve.

If you hear the water continues to run, perhaps your fill valve is leaking. Replace the fill valve.