Toto ultramax II very slow internal leak

[Haff]

I am renovating my bathrooms (slowly). As in many homes one bathroom is above the other. I replaced the toilet in both rooms with the ultramax II one-piece (MS604114CEFG) due in no small part to the reviews I read on this site.
I replaced the upstairs toilet about a month back. and all has seemed well. But a couple weeks ago while seated on the downstairs thrown I heard a slow dripping sound. It took me a while to find the source. I can hear the dripping from the upstairs toilet into the PVC drain. (ceiling of dowstairs is torn out, so pipe is exposed during renovation.
When I open the upstairs tank lid I see the water level is below the overflow level by close to an inch, and when I take the green top off the valve I notice an occasional small amount of water, as if the float was just on the verge of tripping the valve.
I am left with the belief that the flapper leaks, but when I checked the seal I noticed it was completely smooth on the bottom and the mating part on the tank was also completely smooth. As this is a brand new toilet I am surprised it might leak.

Unfortuanately. I ordered the toilet mail order and it sat in my garage for a couple months before install, so I am past my 30 days for a return, and even if I wasnt, the cost to ship it back for what I assume is a minor part fix isnt worth it.

So m question are:
What might be the source of my leak?
What tests can I do to figure it out?

Thanks.

oh, and so it doesnt all sound negative. I love this toilet. Flushes great, comfortable, clean, quick, and my 3 year old isnt in danger from the slow drop lid.

[johnjh2o1]

There a two tests you can do to check the flapper. One is to turn of the water supply to the toilet and see if the water level in the tank drops over time. 2nd is to put some food coloring in the tank and see if it shows up in the bowl. A flapper replacement is a very simple repair. It doesn't mater what make toilet it is they all go bad at some point.
John

[jadnashua]

Make sure that the flapper chain is not kinked and has the specified slack...too much and it can get caught, too little, and it will hold the flapper from properly sealing. Also, that the flapper isn't getting caught up on something. If it is the flapper, Toto will send you one within the warranty period, but in the interim, you could buy one at Lowes or a plumbing supply house.

Terry has indicated that his defect rate on Toto is in the 1% range...far better than any other brand he sells. Nothing manmade is perfect. Some are much less than that goal. This may be a bad flapper, or an adjustment, and is probably easily resolved.

[Haff]

Shut off water, after 10 hours the water level was down to the flapper level and no lower. Felt the flapper surface, smooth as a babys bottom. Felt the white plastic piece it mates to, found a tiny burr. I used my fingernails to try to smooth the burr, today after 10 hours the tnk was still half full. I'll break out some ultrafine sandpaper and maybe a heatgun to try to smooth it better this weekend.

Thanks for the tips.