Tank overfills... is valve bad?

This Toto flushes fine and the tank refills to its previous level. The main fill action seems done but there is a faint hissing sound that continues and gets progressively louder. It appears that the valve is gradually opening again. After a few minutes the plastic tube is passing water from both overflow and directly from the rubber refill tube. I've taken the valve\float assembly apart to clean several times ( it doesn't look dirty ), but this doesn't seem to help. Is the valve bad?

leak

A bad valve would normally be the only way the water could overfill the tank.

I would replace the "assembly cap"

Lowes has them for $1.99

Thanks to both HJ and Terry. Replacing the cap fixed the problem.

Ken