Toto won't stop running

My Toto will not stop running and seems to be getting worse. I have never repaired this type toilet which has the GMAX 528 anti siphon flush valve (as near as I can find).
The only I D for the stool, that I can find, is stamped inside on the back of the tank -- ST743S
Any ideas? I have repaired several of the "old" style but this is all different to me.
Thanks,
Tuskahoma

If it is one with the valve produced by Korky for Toto, you can pop a new cap with seal in it in about 30-seconds for about $2.50 They're available at Lowes, and plumbing supply stores.

It could just be some crud, and you can take it apart and rinse it off, but if I was going to do that, I'd just replace the cap anyways. Check out the Korky site, and see if it is the same valve you have. If not (they use a different model on some), then I'm not familiar with it, but I'm sure there is a fix.

Lowe's did not have a kit (to fix my overflowing Toto toilet) but a kind gentleman there did direct me to Empire Plumbing here in Tulsa. The man there was great. He demonstrated how and then allowed me to practice installing the part.. He even gave me the "cap."
After a couple of mis-steps I got everything back in proper order and now it works correctly again.
jadnashua was correct, the top did pop off after getting it handled correctly - my bad.
Thanks to this forum I avoided a large plumbing bill and got the problem solved.
Gratefully,
Tuskahoma

Lowes here has them, but you were better off going to a good supply house!
I'd keep them in mind for all your plumbing parts needs...
Very nice of him to give you the lesson!