How to Fix a Leaky Toilet Fill Valve

A running toilet is one of those problems that starts as an annoyance and becomes an expensive habit—wasting thousands of gallons a month while your water bill climbs. The fill valve, also called the ballcock, is the mechanism inside the tank that shuts off water once the tank is full. When it leaks or won't seal, water runs constantly from the tank into the bowl, and you hear that faint hiss or trickle even when nobody's using the toilet. The good news is that this is almost always fixable without calling a plumber. You're looking at a straightforward repair that comes down to either cleaning mineral buildup or swapping out a worn valve—both jobs you can handle with a wrench and maybe 45 minutes of your time.

1. Stop the Water Supply. Locate the shutoff valve on the water line behind the toilet, usually a small oval knob or lever on the wall. Turn it clockwise until tight. Then flush the toilet to drain the tank completely. If water keeps trickling in after the flush, the valve is definitely your problem.
2. Detach the Supply Line. Place a bucket or towel under the supply line connection. Use an adjustable wrench to loosen the nut where the line connects to the bottom of the fill valve. Unscrew by hand once it's loose enough. Some water will spill, so be ready.
3. Expose the Valve Seat. Most fill valves have a cap or bonnet on top that unscrews counterclockwise by hand. Remove it and look inside the valve body for sediment, mineral deposits, or debris on the seat where the seal sits. If you see buildup, you've found your leak source.
4. Dissolve Mineral Buildup. Use white vinegar and an old toothbrush to scrub away mineral deposits and sediment from the valve seat and the rubber seal. Soak a cloth in vinegar and wipe everything clean. Rinse thoroughly under running water. If the rubber seal is cracked, torn, or permanently deformed, the valve needs replacement instead of cleaning.
5. Reconnect and Restore Water. Screw the cap or bonnet back onto the fill valve by hand, turning clockwise until snug but not tight. Reattach the supply line by hand first, then use your wrench to tighten the connection nut firmly—you want it sealed but not strained. Turn the shutoff valve counterclockwise to restore water pressure.
6. Verify the Fix Works. Let the tank fill completely and listen. The valve should click or hiss slightly as it closes, then go completely silent. Wait five minutes without flushing. If you still hear water trickling into the bowl or the valve doesn't shut off, the valve seal is worn beyond cleaning and needs replacement.
7. Swap in a New Valve. If the valve still leaks, you'll need to swap it for a new one. With the water off and tank empty, remove the locknut under the tank that holds the valve in place using an adjustable wrench. Pull the old valve up and out from the top of the tank. Insert the new fill valve through the hole, hand-tighten the locknut underneath, and reconnect the supply line. Adjust the water level float on the new valve so the tank fills to the line marked inside the tank.
8. Fine-Tune Water Level. The tank should fill until the water level reaches the line marked inside, usually about an inch below the overflow tube. If your new valve has an adjustable float clip or arm, move it to control fill height. Test by flushing and observing the fill cycle.