A toilet fill valve leaking is usually diagnosed by a small puddle on the floor. The toilet fill valve is connected to the toilet tank via a compression fitting at the bottom of the toilet tank. A rubber seal keeps the water in the tank from draining out. When the fitting loosens or the seal gets old, leaks can occur.
Not sure if this is your problem? See the article, 'Toilet Troubleshooting' for a complete listing of all the toilet topics. Related articles to this one are, 'Fill Valve Repair', 'Toilet Tank Issues', 'Toilet Water Supply Lines', follow these links for more information.
There is only one place that a toilet fill valve can leak and that is at the fitting that connects it to the tank. See the information below for instructions on locating it and tightening it.
A fill valve that is allowing water to run over the top of the fill tube and down into the tank is not really a leak. See the article on 'Adjusting a Toilet float' for information on fixing this problem.
Locating the Toilet Fill Valve Fitting
The toilet fill valve fitting is usually located on the left side of the tank (when you are facing the toilet). There will be a plastic or metal nut directly below the porcelain tank bottom.
You can also identify this by following the water supply line up to the bottom of the tank. The water supply is connected to the toilet fill valve.
Tightening a Toilet Fill Valve Fitting
Shut the water off to the toilet, using the valve at the water supply line.
Take the lid off of the toilet tank. You can flush the toilet at this point to drain most of the water out of the tank. Hold the toilet flapper open to get as much water out of the tank as possible.
With one hand you want to hold onto the base of the toilet fill valve on the inside of the toilet tank. Use the adjustable pliers to tighten the nut on the bottom of the tank. These nuts are usually plastic, so be careful not to tighten.
When you feel that it is tight check for leaks, there should still be some water in the bottom of the tank. If there is not water leaking turn the water to the toilet back on and let the tank fill. Check for leaks again. If there are no leaks you can put the lid back on. If it is still leaking, you can try tightening again.
If the seal still leaks after tightening, you can replace the seal or replace the entire fill valve. See the article 'Toilet Fill Valve Repair' for more information.
Water Supply Line Problems
The fill valve gets blamed for some leaks that are really the fault of the water supply line. There are two fittings that are very close together at the bottom of the toilet tank. One connects the supply line to the fill valve.
See the article, 'Leaking Toilet Supply Lines' for information on what to do with this situation.
Replacing a Toilet Fill Valve
If the fittings are corroded or damaged, you may need to replace the fill valve. This is not an expensive part and they are readily available at home supply stores. See the article, 'Replacing a Toilet Fill Valve' for instructions on getting this done.
The toilet fill valve is attached to the tank with a compression fitting. This fittining may need to be tightened to stop the fitting from leaking. It is not a big job, but it is much easier if you first drain the toilet tank.