That sound is enough to drive anyone crazy. You’re lying in bed at 2:00 AM, the house is silent, and then you hear it—that faint, ghostly hiss of water trickling into the overflow tube. It’s annoying. It’s expensive, too. A toilet that won’t stop running can waste hundreds of gallons of water a day, and if you're on a city meter, you're basically flushing money down the drain. Fix it now.
Most people think they need a plumber the second they hear a leak. Honestly? You probably don't. Most of the time, repairing a toilet that keeps running involves about $15 in parts and twenty minutes of your time, even if you’ve never touched a wrench in your life. The mechanics inside that porcelain tank are surprisingly primitive. It’s a gravity-fed system that hasn’t changed much since the late 19th century. If you understand how the water flows, you can find the leak.
Why your toilet won't stop hissing at you
Basically, there are only three reasons a toilet keeps running. Either the water level is too high, the flapper isn't sealing, or the fill valve is dying. That’s it. To figure out which one is ruining your sleep, you have to take the lid off. Be careful—those lids are heavy, brittle, and slippery. Set it down on a rug or a towel so it doesn't crack.
Look inside. Is the water spilling into that open pipe in the middle? That’s the overflow tube. If water is pouring over the top of it, your fill valve (the tall tower on the left) doesn't know when to quit. If the water level is below the top of that tube but you still hear the hiss, the water is leaking out the bottom of the tank past the flapper.
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The Flapper: The most common culprit
The flapper is that rubber disk at the bottom. Over time, the chlorine in your city water eats away at the rubber. It gets warped, stiff, or covered in a layer of slimy silt. When that happens, it can’t make a watertight seal. Water leaks into the bowl, the float drops, and the fill valve kicks on to replenish the tank. This is what plumbers call a "phantom flush."
Check the chain first. If it's too tight, it keeps the flapper slightly propped open. If it's too long, it can get caught underneath the flapper when you flush. There should be just a tiny bit of slack—maybe half an inch. If the chain is fine but the leak persists, run your finger along the rim of the flush valve seat (the hole the flapper covers). If you feel mineral deposits or roughness, scrub it with a Scotch-Brite pad. Still leaking? Just buy a new flapper. They’re universal most of the time, though brands like Kohler or Toto sometimes require proprietary shapes.
Adjusting the float and the fill valve
If the water is constantly draining into the overflow tube, your float is set too high. This is a common issue in newer homes where the water pressure might be a bit high. On older "ballcock" style valves, you'll see a long metal rod with a floating ball at the end. Literally just bend the rod down a little bit.
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Modern valves usually have a plastic float that slides up and down the main shaft. Look for a long plastic screw. Turn it counter-clockwise. This lowers the shut-off point. You want the water level to be about an inch below the top of the overflow pipe. If you adjust it and the water still won't stop rising, the internal seal in the fill valve is shot. At that point, don't bother trying to take the valve apart to clean the diaphragm. It’s a pain. Just replace the whole assembly with a Fluidmaster 400A or something similar. They’ve been the industry standard for decades because they just work.
The silent leak test
Sometimes you can't see the water moving. It’s a slow, sneaky leak. Grab some food coloring or those blue dye tablets. Drop a few into the tank. Do not flush. Wait fifteen minutes. If the water in the bowl turns blue, you have a leak at the flapper or the flush valve gasket. If the bowl stays clear, your problem is likely a high water level or a faulty fill valve.
It's a simple test, but it saves you from replacing parts that aren't actually broken. I've seen people replace their entire fill valve when all they needed was to wipe a bit of grit off the flapper seat. Waste of time. Waste of money.
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When the problem is actually the handle
This sounds stupid, but check the handle. If the nut holding the handle to the tank is loose, the handle can get stuck in the "down" position. Or, if it's too tight and has some corrosion, it won't spring back up. A little WD-40 on the pivot point or just tightening the nut (remember, it’s a reverse-thread nut—turn left to tighten) often solves the "my toilet runs until I jiggle the handle" mystery.
Identifying a cracked flush valve
This is the nightmare scenario. If you've replaced the flapper and adjusted the float, but water is still disappearing, the actual plastic overflow tube might have a hairline crack. If that tube is cracked, water leaks through the crack and into the bowl. To fix this, you have to take the tank off the bowl. It’s a bigger job. You'll need a set of tank-to-bowl bolts and a new gasket.
- Shut off the water at the wall.
- Flush and sponge out the remaining water.
- Unscrew the bolts under the tank.
- Lift the tank off (it’s heavy, be careful).
- Replace the entire flush valve assembly.
Most people panic at this stage. Don't. It's just nuts and bolts. Just make sure you don't over-tighten the bolts when you put the tank back on. Porcelain cracks easily under pressure. "Snug" is the goal, not "Herculean tight."
A note on "Drop-in" cleaners
If you use those bleach tablets that sit in the tank and turn the water blue, stop. Right now. Plumbers hate them. The high concentration of bleach and chemicals sits against the rubber seals and plastic parts, causing them to degrade at three times the normal rate. If you want a clean toilet, use a clip-on cleaner that hangs inside the bowl, or just use a brush like a normal person. Protecting your gaskets is the best way to avoid repairing a toilet that keeps running every six months.
Actionable steps for a permanent fix
First, do the dye test to confirm if the leak is at the bottom (flapper) or the top (fill valve). If it's the flapper, take the old one with you to the hardware store to ensure a perfect match. If you're replacing the fill valve, opt for a "Pro" version if available; they usually have better seals against debris. Always check your home's water pressure if you find yourself replacing valves frequently—pressure over 80 psi will wreck your plumbing fixtures. Finally, ensure the refill tube (the small rubber hose) is clipped above the water line and pointing into the overflow pipe, not shoved down inside it, which can cause a siphoning effect that keeps the tank running indefinitely.