I had a problem with my toilet's fill valve (it was spraying water all over the top of the tank and it was running down the sides). I recently replaced it with a fluidmaster 400a, but every once in a while it will start filling the bowl a little bit. I don't think water is leaking, but I can't figure out why it keeps re-filling? Has anyone had this problem and know what might be going on? Any help would be great.
If you hear the Fluidmaster cycle and the water level in your tank doesn't continue to rise to the top of the overflow tube, it is almost certain that your tank is losing water. This is most likely through the flapper.
One sure way to test this is to put a few drops of regular old food coloring in the tank, let it sit for a few minutes and see if the water in the bowl is colored. If so, replace the flapper.
Beware, some toilets only work right with O.E.M. flappers. Home Depot and Lowe's sell several different flappers. Try to find one that lists your brand toilet specifically.
If you want to save water there is a great alternative to a Fluidmaster that not only helps to conserve water but it actually helps keep tank clean too. It's called a Hydroclean and you can check it out here: