If your toilet keeps running after a flush, a worn or misaligned flapper is the most common cause. The flapper is a rubber seal at the bottom of the tank that lifts to let water into the bowl, then drops back to stop the flow. When it doesn’t seat properly, water trickles past it constantly and your toilet never shuts off.
Here’s what actually causes that.
Rubber Degradation (the Most Common Culprit)
Flappers are rubber, and rubber breaks down. Chloramine disinfectants (which most California utilities now use instead of plain chlorine) accelerate this significantly. A flapper that’s been in service three to five years is often already past its reliable lifespan, even if it looks okay.
What happens is the rubber warps, hardens, or develops a small crack along the seating edge. It no longer conforms tightly to the flush valve seat. You’ll notice it most as a steady, faint hissing from the tank. The dye-tablet test confirms it: drop a tablet in the tank, don’t flush, and wait ten minutes. If color appears in the bowl, water is leaking past the flapper.
Mineral Buildup on the Flush Valve Seat
In hard-water areas (much of the South Bay and East Bay qualifies), calcium and lime deposits build up on the plastic or brass ring the flapper presses against. Even a new flapper won’t seal against a rough, pitted seat. You can feel this with a fingertip, running it around the seating ring. If it feels gritty or uneven, the flapper isn’t the only problem.
Heavy scale or a visibly pitted seat usually means the whole flush valve needs replacement, which is a job for a licensed plumber.
Chain Length Problems
The chain connecting the flapper to the flush handle lever needs a small amount of slack, but not too much. Too short, and the flapper gets held slightly open even at rest. Too long, and the chain folds under the flapper and prevents it from seating.
The right amount is about half an inch of slack when the lever is at rest. Worth checking before anything else. It costs nothing and takes thirty seconds.
Wrong Flapper for the Toilet
This one trips people up more than you’d expect. Flappers are not universal, even though the packaging sometimes implies they are. Kohler toilets, for example, use a proprietary canister-style flush valve (AquaPiston) on many models, including the Cimarron and San Souci. A standard 2-inch or 3-inch rubber flapper from a big-box store will not work on those. American Standard toilets with a 3-inch flush valve (the Cadet 3 line, for example) also need a 3-inch flapper, not the more common 2-inch.
The model number is molded into the inside of the tank lid or the back wall of the tank. Kohler’s website and American Standard’s website both have parts lookup tools. A licensed plumber will already know which part fits your specific toilet.
How to Diagnose It Before Calling
Take the tank lid off and flush. Watch what happens:
- Does the flapper lift fully and drop cleanly? If the chain snags, that’s your starting point.
- When the tank refills, does the flapper seat and the water stop? If not, watch whether the flapper is floating or just not seating flat.
- Run your finger around the flush valve seat. Smooth or rough?
- Check the flapper material. Is it soft and pliable, or stiff and slightly cracked at the edge?
This takes two minutes and tells you exactly what to report to a plumber: chain issue, worn flapper, or damaged valve seat.
What the Repair Actually Involves
Even a basic flapper swap can go sideways. A wrong part for your specific toilet model means it still runs after the swap. A rough valve seat means even a perfect new flapper won’t seal. And if the flush valve itself needs replacement, that means draining the tank, disconnecting the supply line, pulling the tank off the bowl, and replacing internal components. There’s real potential for cracking porcelain or damaging the tank-to-bowl connection if anything’s overtightened or already corroded.
Getting the diagnosis right before buying anything saves time and money. A licensed plumber will also catch a damaged valve seat that a parts-swap alone won’t fix.
When to Call a Licensed Plumber
If the toilet is still running after you’ve checked the chain and the flapper looks worn, call a plumber. Same if the valve seat feels rough, if you’ve already swapped the flapper and it didn’t fix it, or if you’re dealing with a wall-mounted toilet where the tank is inside the wall.
A running toilet can waste a significant amount of water daily, depending on how bad the leak is. It’s not an emergency, but it’s worth fixing this week, not next month.
Before you hire anyone, verify they hold a current California contractor’s license at cslb.ca.gov. A licensed plumber is legally accountable for the work; an unlicensed one is not.