Fluidmaster 502P21
$6.48 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
See fit and limitsHow to fix toilet phantom flushing
Find the slow leak, then fix it in the right order.
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Quick answer
Phantom flushing means tank water is leaking into the bowl until the fill valve tops it up. Pull the refill tube out of the overflow pipe first, then replace the flapper. If it still leaks, seal the flush valve seat.
For a 2 in. flush valve: Fluidmaster 502P21$6.48 at AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Check it on AmazonThe part for each cause
Each fixes a different cause or flush valve size, so they aren’t scored against each other. All four were new and in stock on Amazon on Oct 24, 2026.
Korky 100BP
$6.84 at Amazon
Check it on AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
See fit and limits
Korky 3060 (3 in.)
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Fluidmaster 555C kit
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Check it on AmazonSee fit and limitsProduct prices and availability are accurate as of the date/time indicated and are subject to change. Any price and availability information displayed on Amazon.com at the time of purchase will apply to the purchase of this product.
Find what’s causing the phantom flush
Run the dye test first, then look inside the tank. The finder follows Fluidmaster’s and Korky’s order of checks.
| What you see | Cause | Fix | Source |
|---|---|---|---|
| Refill tube pushed into the overflow pipe | Siphon from tank to bowl | Clip it outside the pipe | Fluidmaster |
| Color in the bowl, tube fine | Flapper not sealing | Clean, then replace the flapper | Fluidmaster, Korky, EPA |
| New flapper, still leaks | Worn flush valve seat | Seat repair kit or new flush valve | Fluidmaster |
| Chain taut or tangled | Flapper held slightly open | 1 to 2 links of slack | Korky |
| No color in the bowl | Leak outside the tank | Fill valve nut, supply line, tank seals | Fluidmaster |
Korky says the most common cause of a leaking toilet is the flapper, and that it’s easy to blame the fill valve because that’s the part you hear. The fill valve is doing its job, refilling the water that leaked out.
That’s why Korky’s advice is to start at the flapper, at the bottom of the tank. Our flapper vs fill valve guide covers the other kind of running toilet.
What phantom flushing is
Fluidmaster calls it ghost flushing: a refill sound with no one using the toilet.
Fluidmaster says the refill sound can come every few minutes or every few hours, and that it usually means the toilet is losing water, inside or outside. EPA says an old or worn flapper can make a toilet flush on its own or silently leak thousands of gallons a year.
EPA also lists worn toilet flappers first among common household leaks. Fixing household leaks can save about 10% on water bills, per EPA.
Step 1: run the dye test
EPA’s test takes 10 minutes and a few drops of food coloring.
- 1Lift the tank lid
Set it somewhere soft; it’s heavy and breaks easily.
- 2Add color
Put a few drops of food coloring in the tank water.
- 3Don’t flush
Wait 10 minutes without using the toilet.
- 4Look in the bowl
If any color shows in the bowl, the tank is leaking into it (EPA).
- 5Flush right away
EPA says to flush immediately after the test so the dye doesn’t stain the tank.
A meter check confirms it for the whole house. EPA says to read your water meter before and after a two-hour period when no water is being used; if it changes at all, you probably have a leak.
If the dye test is clear but the meter still moves, the leak is somewhere else. Our water leak detector guide covers sensors for hidden leaks.
Step 2: check the refill tube
Fluidmaster lists this check first, because it’s free and takes seconds.
- 1Find the tube
A small hose runs from the fill valve to the top of the overflow pipe in the middle of the tank.
- 2Look at the end
It should clip to the overflow pipe and end above the water, not push down inside it.
- 3Pull it up
If it’s inside the pipe, pull it out and reattach it to the outside of the overflow pipe (Fluidmaster).
- 4Recheck
Run the dye test again after an hour.
Fluidmaster says a refill tube inserted too far into the overflow pipe can cause an internal leak from tank to bowl and “ghost flushing.” Clipping it to the outside of the pipe keeps it from siphoning.
Step 3: clean or replace the flapper
Korky’s steps for a ghost-flushing toilet.
- 1Shut off the water
Turn the valve near the base of the toilet clockwise.
- 2Flush and remove
Flush to drain the tank, then unhook the old flapper from the tabs or ears.
- 3Clean the rim
Clean where the flapper sits to remove sediment and buildup.
- 4Install the new one
Line up the flapper ears with the flush valve tabs and attach it.
- 5Set the chain
Run it as straight up and down as possible, with only 1 to 2 links of slack.
- 6Water back on
Turn the valve counterclockwise, let the tank fill, and test-flush.
Fluidmaster says to wipe the flapper and the flush valve rim first, and replace the flapper only if cleaning doesn’t stop it. EPA says flappers should be checked and replaced at least every five years.
EPA’s tip is to bring the old flapper, or a photo, to the store, and notice whether it has a foam float. Our flapper replacement guide compares more flappers.
2-inch or 3-inch: measure the flush valve
The flapper has to match the flush valve opening. Korky gives an easy way to tell.
| Opening | Korky’s comparison | Typical toilets (Korky) | Flappers here |
|---|---|---|---|
| About 2 in. | An orange or a baseball | Most toilets, per Korky, including most made before 2000 | Fluidmaster 502P21, Korky 100BP |
| About 3 in. | A grapefruit or a softball | Popular since 2000, per Korky | Korky 3060 |
Korky warns that Kohler one-piece toilets need a specialty flapper. Our 2-inch vs 3-inch flapper guide covers sizing by brand.
Step 4: if a new flapper still leaks, fix the seat
The flapper seals against the rim of the flush valve, called the seat. A pitted or cracked seat leaks under any flapper.
- 1Feel the rim
With the tank drained, run a finger around the seat for pits, ridges or a crack.
- 22 in. valve, sound body
Fluidmaster’s 555C: peel the paper off the sealant ring and stick it on the old seat; no glue, no tank removal.
- 3Cracked valve
Korky says a cracked or damaged flush valve needs replacing.
- 4Replacement valve
Fluidmaster recommends its K-507A-008 for 2 in. toilets and its 540AKR for 3 in..
Fluidmaster’s 555C kit comes with its PerforMAX adjustable flapper, and Fluidmaster says it’s best for 1.28 to 3.5+ gallon-per-flush toilets with 2 in. flush valves. Replacing the whole flush valve means taking the tank off the bowl, which is a bigger job. Our still-running-after-a-new-flapper guide covers that case in more depth.
If the dye test is clear: leaks outside the tank
Fluidmaster says the same refill sound can come from water escaping outside the toilet.
| Where the water shows | What to check (Fluidmaster) |
|---|---|
| Behind the toilet, at the back | The bottom of the fill valve, the supply line, or the tank-to-bowl seals on a two-piece toilet |
| Dripping from the bottom of the tank | That the fill valve locknut under the tank is tight against the china |
| Still dripping | Remove the fill valve and clean the seal area |
A drip outside is easier to find than a silent one inside: lay a dry paper towel under the tank and supply line overnight. Fluidmaster says it’s always a good idea to replace the supply line if it’s older than 5 years. Our toilet supply line guide covers which to buy.
Step 5: make sure the phantom flush is gone
A phantom flush is intermittent, so one quiet minute doesn’t prove it’s fixed.
- 1Dye test again
Repeat EPA’s food-coloring test after the tank has refilled and settled.
- 2Listen overnight
Fluidmaster says the refill sound can come every few minutes or every few hours; a quiet night is a good sign.
- 3Read the meter
EPA’s two-hour meter check with no water in use tells you if any leak is left in the house.
- 4Mark the water line
Draw a pencil line at the tank water level and check it in the morning.
My call: a pencil line is the simplest leak detector there is. If the level drops overnight and the dye test shows color, the flapper or seat still isn’t sealing. If the level holds but you still hear refills, look outside the tank.
Korky’s point applies here too: the fill valve you hear is usually just replacing water lost somewhere else.
Parts, fit and limits
From Fluidmaster’s and Korky’s product pages.
| Part | Key specs (maker) | Fixes | Pick it if | Skip it if |
|---|---|---|---|---|
| Fluidmaster 502P21 | Universal adjustable flapper, solid frame, for 2 in. flush valves; Fluidmaster’s pick for 2 in. | Flapper leak | Your valve is 2 in. | Your valve is 3 in. or a Kohler one-piece |
| Korky 100BP | Universal 2 in., adjusts for 1.28 to 5 gpf toilets, chlorine-resistant rubber, made in USA, 5-year warranty | Flapper leak | Your valve is 2 in. and you want Korky’s warranty | Your valve is 3 in. |
| Korky 3060 | Universal 3 in., adjustable, for 1.6 and 1.28 gpf toilets since 2005, 5-year warranty | Flapper leak | Your valve is 3 in. | Your valve is 2 in. or a Kohler one-piece |
| Fluidmaster 555C kit | Peel-and-stick seat ring plus PerforMAX flapper, 2 in. valves, 1.28 to 3.5+ gpf | Worn seat | A new flapper still leaks on a 2 in. valve | The flush valve is cracked |
Korky’s flappers adjust with a band under the flapper. Korky says turning it to higher numbers closes holes so the flapper stays open longer, if you have to hold the handle for a full flush. That adjustment fixes a weak flush after a flapper swap, not a leak.
When to do it yourself and when to call a plumber
DIY
The dye test, the refill tube, a flapper, a chain.
DIY with patience
Fluidmaster’s 555C seat kit on a 2 in. valve.
Call a plumber
A cracked flush valve that means pulling the tank.
Plumber job
Water running down the back of the base: Fluidmaster says that points to failed tank-to-bowl seals, a tank-off job; Oatey lists a cracked tank as a cause too.
The first three steps need no tools. EPA notes that a local hardware store or a licensed plumber can help if you get stuck. Our running toilet guide covers a toilet that runs all the time instead of now and then.
Questions people ask
Why does my toilet flush by itself?
Water is leaking from the tank into the bowl, so the fill valve refills it. EPA says a worn flapper can make a toilet flush on its own.
Is phantom flushing dangerous?
No, but it wastes water; EPA says a worn flapper can silently leak thousands of gallons a year.
How do I test for a phantom flush?
Put a few drops of food coloring in the tank and wait 10 minutes without flushing; color in the bowl means a leak (EPA).
Can the refill tube cause ghost flushing?
Yes. Fluidmaster says a refill tube pushed too far into the overflow pipe can leak tank water into the bowl; clip it outside the pipe.
How often should I replace a toilet flapper?
EPA says to check it periodically and replace it at least every five years.
Why does my new flapper still leak?
The flush valve seat may be worn or the chain too tight. Fluidmaster’s 555C seals a worn 2 in. seat; Korky says 1 to 2 links of chain slack.
Do I need a 2-inch or 3-inch flapper?
Measure the flush valve opening: about 2 in. (orange-sized) or about 3 in. (grapefruit-sized), per Korky.
Why does my toilet run for a few seconds every hour?
Fluidmaster says intermittent refill sounds, every few minutes or every few hours, usually mean the toilet is losing water inside or outside the tank.
Can a bad fill valve cause phantom flushing?
Yes. Oatey lists a malfunctioning fill valve among the causes, since it can leave the tank at the wrong water level. Korky says the flapper is the most common cause, though, and the fill valve you hear is often just refilling water that leaked out.
Will a universal flapper fit a Kohler toilet?
Not a Kohler one-piece; Korky says those need a specialty flapper and its kits won’t fit.
How we checked these fixes
Why trust me
I’m Isaac Daniels, a licensed journeyman plumber in Georgia (license #JP507711). I’ve used every part on this page. The steps come from Fluidmaster’s and Korky’s troubleshooting pages and EPA’s leak guidance; my own advice is marked “my call.”
How I pick
- Each part fixes a different cause or valve size, so they’re a kit, not scored against each other.
- Fit facts come from the maker’s pages.
- Genuine maker parts only, new and in stock on Amazon on Oct 24, 2026.
- No plumbing services are offered here.
Sources
- U.S. EPA WaterSense, Fix a Leak Week: checking for leaks and toilet leaks (epa.gov/watersense/fix-leak-week).
- Oatey, “How to Stop Ghost Flushing” blog guide (oatey.com).
- Fluidmaster, Fix toilet ghost flushing and refill sounds troubleshooting guide; 555C 2 in. flush valve repair kit page (fluidmaster.com).
- Korky (Lavelle Industries), How to fix toilet flushing on its own; What size flapper do I need; 100 and 3060 flapper pages (korky.com).
- Product listing details (Amazon Creators API, Oct 24, 2026).
- Search Console data for this page, Jul 24 to Oct 21, 2026 (pulled Oct 24, 2026).