Feel where the seal sits to close off the water...it may have some crud on it or be rough. All it may take is to rub your finger around the seat to clean off any crud. If the seal isn't seated properly in the slot, it may get distorted. If the seat isn't smooth, you may need a new flush valve. Also, depending on its storage and age, the gasket may be a little hard...sitting with water pressure on it may straighten it out. The only other thing I can think of is if the seal where the seat attaches has degraded. To fix that, you need to take the tank off, remove the flush valve, and then replace the seal. You'd also want to replace the washers under the head of the bolts holding the tank to the bowl. Once you're done, it should be as good as new. That may or may not be great, but it should last a long time.