After fifteen minutes, Kohler toilet water tank keeps overflowing.

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
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Frank wrote:

You may have exceptionally high water pressure, or the "seal" (special rubber washer) in the ballcock may be at end of life. IIRC, Kohler used the Fluid Master ballcock: http://www.fluidmaster.com/usa.html
If it looks like that, replace the seal.
Jim
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
If it is Fluidmaster, you can turn off the water, twist the top half off and turn on the water to try to clean it out. They also sell some replacemnet parts if you don't want to replace the whole thing.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Frank wrote:

Bad fill valve. You may be able to replace the internal valve parts, or just replace the whole thing.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Frank wrote:

floating.. on one of my old toilets the ball was full of water and heavy as hell and would sink down and not turn the water off as it would not float all the way to the top of the water....
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.