The function of an anti-siphon toilet fill valve is to prevent the back flow of water from a toilet tank into a potable drinking water system. It is imperative to your health and the heath of the general public to keep an anti-siphon valve in good working condition at all times. Learning how to repair a leaking anti-siphon valve on a toilet--such as the Fluidmaster 400A toilet fill valve--will only take a few minutes; the repair itself should take the novice DIY repairperson about thirty minutes or less.
Follow the fill valve water supply line down from the left side of the toilet to the water shut-off valve. Turn the valve's handle to the right and shut it off. Flush the toilet to make sure the water has stopped completely.
Remove the toilet tank lid carefully, and then set the lid on a soft surface, away from the work area.
Lift the short fill valve lever that protrudes from the top of the fill valve, then turn the lever towards the back of the toilet tank. This will unlock the fill valve top cap.
Hold the top cap over the fill valve with one hand, then turn the water valve on with the other hand (just enough for the water to flow through the valve). This will remove any sediment and debris from the fill valve. Turn the water back off.
Turn the top cap over: You will see a black rubber valve seal. Remove the seal with your fingers, then install a new one in the same position.
Set the top cap with the new seal back on top of the fill valve, then rotate the short lever towards yourself. This will lock the top cap in place.
Turn the water valve back on again and wait for the toilet tank to fill with water and shut off.
Adjust the water level setting, if necessary, to the manufacturer's recommended setting. Use a screwdriver to turn the screw located at the lower end of the short lift arm to the left to lower the water level, and to the right to raise the water level.
Flush the toilet and wait for it to fill each time you adjust the water level.
Replace the toilet tank lid carefully.
You can also cover the top of the fill valve with a cup or glass when you turn the water on, in order to clean the fill valve.
The toilet tank lid is made of porcelain; it will break easily if dropped.