I have a toilet that flushes properly, and then several hours later the tank is empty. The toilet is not running. I have a Korky Brand unit inside the toilet that has a up and down valve assembly that fills the tank. I also use a Fluidmaster bleach type bowl cleaner that is attached to the back of the tank and fills with bllue water when flushed. This is how I know the tank is empty when the toilet is flushed it fills with blue water, and hours later the water is clear. What is causing this? I have been on the internet for a while now and all the problems seem to be about running toilets. Mine is not running, it just mysteriously empties.

The first thing I would do is change the flapper. While you're down there run your fingers around the seat. If you find any nicks or rough edges tell me about it. Good luck, Tom