diagnose this leak ?

I found water slowly seeping between the bottom of the toilet tank and the flush valve rubber gasket.

I looked at the flush valve, and it looks solid and tight. This toilet was installed last month.

cause? what should I look for ?

The bolts attaching the tank to the bowl may have been tightened unevenly, causing the tank to tilt down slightly on one side. Or the bolts may have been OVERtightened, causing the gasket to distort (as well as increasing the risk of the tank cracking).

The tank mounting bolts should be evenly torqued and only snug-tight.

Remove the tank and tighten the large nut on the flush valve. Many times they are not tightened enough form the manufacture. It's something we always do before installing a new toilet.

John

Since you have to remove the tank anyway, take it outside and fill with water. You will spot the leak. Are you sure it is not coming from the bolts? I always nut the bolts directly at the bottom of the tank. That makes a very tight, secure seal on the bolt gaskets, and the second nut to hold the tank onto the bowl only needs to be hand tight.