Vacuity keeps on running ...

I have 5 Briggs Vacuity toilets in my house and they are all about 7 years old. Two of the 5 periodically "run" unattended--the water level in the tank slowly drops to the point that the float drops and the tank refills to the top level.

I have replaced the flapper with a genuine Briggs repacement (I too have discoved that other brands do not work well) and cleaned up the seats--they do not appear to be leaking.

Where else could there be a leak? (floor is dry)

It could be one of these seals leaking.

leak

It won't be the gasket between the tank and bowl.

My guess is if it isn't the flapper inside, you probably have a crack in the plastic flush valve assembly. It's that white stand pipe the flapper is connected to.

I haven't seen a brass flush valve in a long time. Is cheaper better?