"Toto"ly frustrated with my leaky Toto toilet!

[forester7]

Hello. I have two 10-year old Toto toilets (CST744SL-01 Drake) that have both recently started hissing. I know the sound is coming from the fill valve passing water. I was told to try changing the cap on the fill valve. I tried this on one and it did not work. I then tried replacing the entire fill valve with suggested replacement (Korky genuine Toto fill valve). If anything, it is now working worse. Once the reservoir fills, it shuts off, but shortly starts seeping water in again. I now notice that the reservoir water level slowly drops after it fills. Is that likely causing the water to start seeping in the fill valve again? If so, what could the issue be? I have hard water and cleaned up a little mineral buildup on the flapper, but that did not help. Could I need a new flapper? If so, what would cause a rubber flapper to go "bad"? Thanks in advance for any help!

 

[Twowaxhack]

Change the flapper

 

[Geofd]

To get the correct flapper for your toilet there are 4 #s in black on the right side inside the tank any plumbing supply house will have it if you choose hd/or lowes make sure the brand and flapper work with toro

 

[forester7]

I replaced the flapper and it appears to be working now! Thank you to everyone for your assistance!