Most of the Toto toilets come with a valve made for them by Korky. If they have that one, the only real wear part in the refill valve is the seal cap, available at Lowes and most hardware stores. It takes about a minute to replace with no tools. That will probably resolve this. But, as long as it doesn't overfill, and does refill the toilet, you can live with it for a long time. The instructions are on the package. Basically, turn the water off, pop the cover off the top of the refill valve, twist the seal cap, remove the cap, put a new cap on, put the cover back on, turn on the water. Details are on the package...cost about $3. This is true for any toilet that uses the Korky refill valves.