The only thing you can replace in the Fluidmaster is a gasket...it could be clogged up with crud. The instructions are on their website...basically, you twist the tower (make sure you've turned the water off first!). Testing if the flapper is leaking is easy...shut the water supply off then leave it...see if the tank empties. An alternative is to put some food coloring in. If it is leaking you'll see the color appear in the bowl. But, if it leaks fast, and you don't check it out soon enough, it could have washed all of that down the drain, but then both the tank and bowl would be clear.