Toto Leaky Fill Valve

I recently installed a Toto UltraMax toilet. The toilet works fine. However, the fill valve starts to leak after a week or so. It will shut the water off at the appropriate level. Then a slow leak will occur and the water level will slowly rise until it leaks into the overflow tube after 10 to 15 minutes. I have replaced the fill valve twice, but all three fill valves have this problem. The problem does not begin until the fill valve has been in the toilet for more than a week. I have tried to adjust the height level ever so slightly, but this does not stop the leak.

What is the problem?

How do I fix it?

Thanks.

Are you on a well? You might have crud (sand, etc.) that is getting into the gasket. You can replace the cap which contains the seal for about $2, and while you have it off, check it for crud.

Instructions here

These valves work fine with any legal water pressure.
Anything over 80 PSI needs a pressure reducer.

You might want to try a different brand of fill valve. Could be your water pressure is too high for this one.