Toilet tank draining

[StPeteDR]

I have a Kohler San Raphael one piece toilet. The toilet was running intermittently and when I looked in the tank, the flapper was not seated properly because the flush valve gasket had expanded from age and was touching the tip of the flapper, preventing it from properly sealing over the flush valve drain opening. I bought a new gasket, which I had replaced previously. I recall how difficult it was to reinstall the flush valve with the new gasket so I was looking for tips on how to make the job easier. I found a YouTube video where the person compressed the gasket between two porcelain tiles with clamps and froze it. He then put it on the flush valve and it was an easier install. I tried that and it was a much easier install. The intermittent filling stopped for a while but came back.
I've examined the gasket and I can't see any visible damage to it and the texture/shape does not seem to be altered. However, I'm wondering if freezing somehow compromised the integrity of the gasket? I've replaced all of the parts in the the tank and the toilet is still running intermittently. If I shut the water off, the tank will drain down to the flapper.
I'm looking for any thoughts on this or any additional advice on how to solve this issue. Any help is greatly appreciated.

Thanks,
Dennis

 

[Peterson]

If the tank drains down to the flapper, and there's still water in the tank below the flush valve seat, it's the flapper that is leaking.

I doubt freezing the gasket would have done anything to it. Maybe the plastic flush valve pipe might have a crack in it?