Upon returning from vacation, I noticed the upstairs toliet was leaking (refill valve came on about every 5 minutes). I put some dye in the tank and saw it come out the bottom of the bowl. Also, after filling the tank and turning off the water suppy, I could almost watch the water level drop. So I replaced the flush valve (entire vale, not just the stopper) and the rubber washer that goes between the tank and the bowl.
But the leak continued. So I removed the tank, put the bolts back in, filled the tank with a hose and let it sit for 30 minutes. No leaking. I then placed the tank back onto the bowl, connected the bolts, but didn't connect the water supply line, and waited about 20 minutes. No leaking. Then I connecetd the water supply line, but left the water turned off, and the leaking began again. I thought that perhaps the tank was flexing just a bit when the line was hooked up, so I tried adjusting the tank bolts without cracking the tank, but the leak continues. I haven't found any cracks in the tank, but maybe there's a small one in there somewhere I can't see.
For now, it seems like I should just replace the tank.
Anyone have some ideas as to what the problem is?