Toto CST854; replaced flapper but water keeps running on after flushing

[VARYA]

Hello, I have a 20-odd year TOTO CST 854 and the water now keeps running after flushing, so I have to turn the valve off. I replaced the flapper with a TOTO-like model, but problem remains. Any advice?

 

[Terry]

terrylove.com/korky

The fill valve many be bad. The link above has some steps to look at.