Hi all I just replaced the fill valve in my toilet with a Fluidmaster 400a. Everything seemed fine at first. The tank would fill to about a half inch below the overflow tube and then stop. However, after a period of a few hours, I came back into the bathroom only to find the toilet running. I removed the tank lid and could see that the tank was overfilling and allowing water to flow into the overflow tube. I flushed the toilet, water then filled the tank to a half inch below the overflow tube and stopped. A few hours later, the toilet was running again and water was going into the overflow tube. What could be causing this? Why is the valve shutting off at the proper time, then engaging later and never turning off?