I already owned a Toto Guinevere Toilet. It was the 1.6 gallon flush. I was surprised by your review and the dripping sound you encountered. I was considering buying a second one, and despite your review, I went ahead. The supplier advertised that the 1.6 gallon was still available, but sent the 1.28 gallon instead. I decided to keep it, although reluctantly.

To my surprise, after I installed it, I too heard a constant dripping. To confirm water was leaking, I shut off the valve under the toilet feeding water, made a mark with a Sharpie at the water line, and let it sit for several hours. After about 6 hours, the tank was empty. All the water leaked out. I tried reseating the flapper and tightening the brass screws that mount the white plastic seat that the flapper sits on. No difference.

I called Toto customer service. They claim not to have heard of the issue, but sent me both a new flapper and a new seat which included the vertical fill pipe. First, I just installed the flapper. Not change, drip was still there. Then I removed the old filler seat and installed the new one. After the first flush, I did hear the dripping sound. But after a few minutes, it was quite. To confirm the problem was solved, I again closed the water fill valve and waited for about 6 hours. This time, the tank remained full.

The dripping for the most part is fixed. The toilet does seem to drip for a couple of minutes after a flush. Seems like the toilet slightly overfills and this must drip out. (My 1.6 gallon Guinevere does not do this.) It will also drip if new waste is added to the bowl without flushing. I consider this normal and it stops dripping after the excess water drips out. The constant dripping is gone.

I hope this helps anyone out there with the dripping problem.