Frank wrote:
I have a 1.6 liter per flush Kohler toilet - about five years old.
This is my problem. After I flush, all goes well. The water tank
fills up and stops filling as it's supposed too. This about fifteen
minutes later, however, the water tank slightly overfills and thus
begins to pour into the overfull hole. This is very gradual. I put a
new flapper on it and it is not leaking. Seems like the float level
is OK as well because it stops short of overflow when toilet is
initially flushed.

What is causing the slow overfilling of the water tank and how can I
fix it?

Any assistance is greatly appreciated.

Frank

Bad fill valve. You may be able to replace the internal valve parts, or
just replace the whole thing.