A new flapper can still leak. Check the chain, seat and refill tube first. Water spilling into the overflow points to the fill side; consider Fluidmaster 400H only after adjustment and cleaning fail.
Fill-side fault only: Fluidmaster 400H$14.97 at AmazonPrice as of Oct 24, 2026, 5:30 AM EDT
Find where the water goes after the flapper change
Gravity-tank routing: inspect the new flapper at the bottom, the refill tube above the waterline and the overflow opening. Diagram is schematic, not an installation-height drawing.
Tank fills, stops, then refills
The fill valve may be replacing lost tank water. Test the new flapper’s seal and refill-tube position before replacing the fill valve.
Water spills over the overflow opening
Check the fill valve’s shutoff level and float clearance. Adjust or service the identified valve before choosing a replacement.
Water outside the tank or a pressure vessel inside
Stop this repair path. Isolate an external leak if safe and call a licensed plumber. Pressure-assisted systems need their own maker’s service instructions.
Start with the lid off and watch a complete refill. Set the porcelain lid somewhere stable before reaching into the tank. The useful clue is what happens when the tank should stop filling: a rising waterline, a falling waterline between cycles, or water outside the tank.
The noise alone does not identify the faulty part. Korky’s running-toilet instructions explain that a working fill valve can keep replenishing a leak through the flapper or flush seal. A new flapper changes that part’s age; it does not prove its fit, chain adjustment or seating surface is right.
Mark the tank level and isolate the supply
1
Fill and mark
Let the gravity tank refill, then mark its waterline with a pencil. Korky’s leak test starts from a filled tank. Do not flush during the waiting period.
2
Close the toilet’s water supply
Close the fixture shutoff and confirm that incoming water stops. If the valve leaks, will not isolate the supply or moves the pipe, stop and call a licensed plumber before disassembly.
3
Wait at least 25 minutes
Compare the waterline with your pencil mark. Korky specifies at least 25 minutes. A lower level means the tank is losing water; check outside the tank as well as the bowl.
4
Use the stopping level as a clue
If there is no external leak, Korky extends the observation for a couple of hours. Water settling near the flapper or seal directs attention there. A level at an overflow defect or the tank bottom directs attention to the flush-valve assembly.
A falling waterline locates a problem to investigate; it does not prove that only one part is faulty. If the shutoff still lets water in, the tank is not isolated and this test cannot be read reliably.
Keep this test separate from an ordinary flush. After flushing, the tank is supposed to empty and refill. Here, you are asking whether stored water disappears while the toilet is unused and the supply is closed. Write down the starting level and the later level rather than relying on the sound.
Correct the new flapper’s installation first
The chain must allow the flapper to rest on its seat. Korky’s guide calls for 1–2 links of slack; Fluidmaster’s documented 400H troubleshooting example uses about 1/2 inch. Follow the installed flapper’s instructions.
1
Shut off and drain before resetting the flapper
Korky’s procedure closes the toilet supply and flushes the tank before removing the flapper. If the shutoff does not work, leave the connections alone and use a licensed plumber.
2
Check the opening and mechanism
Match the replacement to the tank model and flush opening. A hinged flapper and a tower or canister seal are different repair parts. Use the size guide below if the opening is uncertain.
3
Clear the sealing surface
Remove sediment and debris where the flapper sits, as Korky directs. Inspect the exposed seat for damage. Do not try to cure a damaged seat by pulling the new flapper down harder with the chain.
4
Reset the mounting and chain
Seat the flapper ears on their intended tabs and route the chain as nearly vertical as the installed model allows. At rest, the chain must not hold the seal open or lie underneath it. Use that flapper’s own slack setting.
5
Refill and repeat the test
Reopen the supply and perform a test flush. Watch the flapper return to its seat, then repeat the isolated waterline test if the tank still refills between uses. A quiet single flush is not the same observation as a stable tank level.
A flapper labeled universal still needs the right flush opening and tank application. Do not choose by the outside width of the rubber alone. Our 2-inch versus 3-inch flapper guide explains the size distinction; the tank number and maker’s fit list settle the model match.
If the new flapper closes before a useful flush finishes, check its adjustable flush setting and chain instructions. Tightening the chain until the flapper hangs open creates a different fault. Keep the sealing check separate from flush duration so an adjustment for one does not conceal the other.
My call: if resetting the chain and clearing the seat leave the tank level stable, keep the new flapper. Another repair part has no diagnosed job to do.
Check the refill tube before blaming either valve
For an open-overflow gravity tank, use the prescribed clip rather than pushing a loose refill hose beneath the tank waterline. Some canister systems have dedicated connections; follow their drawings instead.
The small refill tube supplies bowl-refill water through the flush valve. Flow during the tank’s refill can be normal. Persistent flow after the tank should have shut off, or repeated refill cycles without a flush, needs investigation. Do not label every stream from this tube a leak.
Fluidmaster’s 401LGR instructions warn against pushing the refill tube down the overflow because it can waste water. Its PRO45C instructions describe the siphoning problem. Check whether the tube moved during the flapper change. With the supply off, restore the connection shown for the installed valve and tank.
Do not apply the open-overflow clip drawing to every canister. Fluidmaster’s 400H booklet shows separate connections for Mansfield, Kohler AquaPiston and American Standard Champion 3. A dedicated nipple or refill hole needs that arrangement. Leave a pressure-assisted vessel and its tubing to its own service instructions.
Repositioning an existing refill tube can be the complete repair. After correction, refill the tank and repeat the supply-off observation; do not buy a fill valve solely because its refill hose was in the wrong place.
Service a confirmed fill-side fault
Float rubs the wall or lever
Correct clearance under the valve’s instructions. The Fluidmaster 400H booklet requires the float to clear the tank wall, lever and flush valve.
Waterline reaches the overflow
Lower the setting using the identified valve’s adjustment. For Fluidmaster 400H, counterclockwise lowers the tank float; clockwise raises it. Flush and recheck.
Correct setting, but valve still runs
Follow that model’s debris-service procedure. If service does not restore shutoff, compare a replacement’s height and tank clearance below.
1
For a documented 400H, isolate and flush
Fluidmaster’s 400H booklet starts cap service by turning off the supply and flushing the toilet. Never remove the cap while the valve is pressurized. Use the booklet’s grip and cap-release drawing for this model.
2
Shield the uncapped valve with a cup
The 400H procedure holds an empty cup upside down over the valve before cycling the supply on and off a few times to clear debris. Keep the cup over the opening to control splashing.
3
Shut the supply again before recapping
Fluidmaster then closes the supply, aligns the cap assembly and locks it in place. Reopen only after reassembly. If you cannot control the supply or identify the model, use a licensed plumber instead of copying these steps onto a different valve.
The 400H manual also documents a serviceable internal seal. A serviceable valve does not automatically need complete replacement. Confirm the installed model and its service instructions first; the two complete valves below are for a fault that adjustment or appropriate service has not solved.
If you replace the whole fill valve, Fluidmaster’s 400H procedure shuts off water, flushes and sponges out the remainder before disconnecting the supply and old valve. Its locknut and supply coupling are hand-tightened only. Overtightening can crack the valve or tank and cause flooding. Check for leaks after reopening the supply.
Fill valves for a fault the tests confirm
Choose one only after checking tank loss, refill-tube routing and the existing valve’s adjustment. Compare height ranges and float clearance before buying.
Product 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.
Check the replacement’s fit before buying
Best for separate tank and bowl adjustments
Fluidmaster 400H fill valve
Product photo: Amazon
Fluidmaster 400H PerforMAX toilet fill valve, 1-pack
Booklet includes debris service and cap reassembly
Cons
Does not replace a flapper or flush-valve seat
9-inch minimum height still needs a tank-fit check
Canister refill connections differ by mechanism
Model 400H9–14 in. adjustable heightTank screw + bowl dial1 valve7-year warranty
Buy it if
The valve will not shut off after correct adjustment and appropriate service, and the 9–14-inch range and clearances suit your tank.
Skip it if
The isolated tank loses water through its new flapper, seat or overflow. Find that leak before replacing the fill valve.
The Fluidmaster 400H belongs on the fill side of this repair. If the tank water rises into the overflow after the float should shut the supply, check adjustment and debris first. If those steps do not restore shutoff, this complete valve is one replacement option for a compatible gravity tank.
Its useful distinction is separate control of tank and bowl water. The tank adjustment screw changes the shutoff level, while the bowl dial changes refill delivery. These controls address different observations, so a low bowl does not require raising the tank until water spills over the overflow. Start with the toilet’s prescribed tank waterline.
Fluidmaster lists adjustable valve height from 9 to 14 inches. That is a fit range, not a promise that every tank accepts it. Check lid clearance and room for the float to travel without touching the tank wall, lever or flush valve. A tall enough tank can still have an obstructed float path.
The 400H booklet recommends tank water roughly 1/2 inch below the overflow top. For this model, clockwise raises the tank float and counterclockwise lowers it. Flush after an adjustment and observe the final level. Do not apply those directions to an unidentified valve with a different adjustment mechanism.
Installation begins with the supply off and tank drained. The booklet calls for hand-tightening the valve locknut and supply connection, then checking for leaks after reopening the water. It warns that overtightening can crack components and cause flooding. A shutoff that does not isolate the tank needs a plumber before this swap.
My call: choose the 400H when a confirmed fill-valve fault remains and you want its separate tank and bowl controls. Keep the new flapper if it passes the leak test. Replacing a working flapper again will not improve a fill valve’s ability to shut off.
Model 528X8-1/2–14 in. heightInternal floatRefill adjuster included10-year warranty
Buy it if
You have confirmed a fill-valve fault, checked the tank application and need a valve whose stated range starts at 8-1/2 inches.
Skip it if
A chain adjustment or refill-tube correction stops the running, or the isolated tank still loses water through the flush side.
Korky’s 528X is another complete fill-valve option after the leak path has been identified. Its stated height starts at 8-1/2 inches and extends to 14 inches. That lower starting point is a reason to compare it with the 400H in a shorter tank, although height alone does not establish compatibility.
Korky’s product page describes an internal float and a serviceable strainer that catches sediment. The included parts list names the refill tube and clip, installation hardware and a refill adjuster. Those details matter when the old valve has damaged hardware as well as a shutoff fault. Identify what is actually failing before choosing a complete replacement.
For a new Korky valve that will not shut off, the maker directs you to check its water-level mark at 1/2 inch below the overflow top. Its guidance explains that changing valve height changes tank level because the float is internal. Follow the 528X setup drawing rather than turning a screw copied from Fluidmaster instructions.
The refill adjuster controls bowl-refill delivery. Check the bowl level after a complete flush and refill, separately from the tank’s shutoff level. Use the 528X instructions for its adjuster rather than the 400H’s bowl dial.
Korky describes the 528X as a broad-fit valve, but a broad fit claim is not confirmation for your tank number. Check the current maker instructions, the toilet’s required waterline and the space beneath its lid. Keep the refill connection matched to the toilet’s flush mechanism, especially if it uses a canister instead of an open overflow.
My call: compare the 528X when its lower stated height or serviceable strainer suits the tank. Do not choose it solely to silence refill cycles caused by a leaking flapper. Isolate the supply before replacement; if it cannot be shut off safely, have a licensed plumber handle the repair.
Listing height (7.75–13.5 in.) differs from Korky’s
Not a repair for a leaking flapper seat
Model 528MP8-1/2–14 in. heightInternal floatRefill adjuster included5-year warranty
Buy it if
You’ve confirmed a fill-valve fault on a toilet made since 1994 that uses 1.6 gallons per flush or less.
Skip it if
The toilet predates 1994, or the isolated tank still loses water through the flush side.
Korky’s 528MP QuietFILL Platinum is the third option once the tests point to the fill side. Korky says it’s designed for toilets made since 1994 that use 1.6 gallons per flush or less, including high-efficiency (HET) models. For an older tank, the 528X or the 400H is the closer match.
Its stated height runs from 8-1/2 to 14 inches, the same as the 528X. The Amazon listing gives 7.75 to 13.5 inches instead. We go by Korky’s figure; measure your tank and read the current instructions before you set the height.
The refill adjuster is why it suits low-flow toilets. Korky says it delivers the right amount of water to the bowl to keep the toilet’s original performance. Set it after a complete flush and refill, separately from the tank level.
Like the 528X, it has a strainer that catches sediment so the valve can be serviced, and Korky’s valves use an internal float, so changing the valve height changes the tank level. Korky’s check for a new valve that won’t shut off is the same: the water-level mark sits 1/2 inch below the top of the overflow tube.
Korky calls it the quietest fill valve in its lab testing. We haven’t measured noise, so that claim isn’t in our score. The warranty is 5 years, half the 528X’s 10, and Korky says both are made in the USA.
In the box: the valve, a refill tube with a metal clip, the refill adjuster, installation hardware and instructions.
My call: pick the 528MP for a 1994-or-newer low-flow toilet once the dye test and the refill-tube check point to the fill valve. If the isolated tank still loses water at the seal, it won’t help; fix the flush side first.
Treating the new flapper as the cause of overfilling
Isolated tank loses water near the seal
Check flapper fit, seat and flush assembly
Using a complete fill valve as a seal repair
Water appears outside the tank
Safely isolate and call a licensed plumber
Continuing a bowl-leak test while the floor gets wet
What a dye test can and cannot prove
EPA’s household test adds a few drops of food coloring to the tank and waits 10 minutes. Color reaching the bowl indicates a leak. Flush afterward to avoid staining. Perform the observation without flushing during the wait, or you will deliberately send tank water into the bowl.
A positive dye test after a flapper change means tank water still reaches the bowl. It does not prove the replacement rubber itself is defective. Check whether the tank is overfilling, the chain is holding the seal up, or the refill tube is routed incorrectly before condemning the new part.
Test
What it tells you
What it does not settle
EPA dye test: 10 minutes
Colored tank water reaches the bowl
Exact component or whether several faults coexist
Korky isolated-level test: at least 25 minutes
Tank loses stored water with supply closed
Precise opening without checking the stopping level and exterior
Observe a complete refill
Water rises into overflow or shuts off below it
Whether a slower leak remains between flushes
If both the flapper and fill valve are new, go back to installation rather than assuming two new parts rule out their systems. Check the new valve’s height and shutoff setting. Then verify the refill connection and repeat the isolated-level test with the new flapper seated. A changed part can introduce a changed adjustment.
Keep operating water level separate from valve installation height. The 400H booklet’s roughly 1/2-inch gap below the overflow describes stored tank water. Its separate cap-height drawing governs valve installation. Neither measurement is a reason to ignore the toilet’s marked waterline or the supplied instructions for a different valve.
Potential confusion
Evidence to use
Decision
400H listing title has an incomplete height range
Fluidmaster product page: 9–14 in. adjustment
Use maker’s complete range; listing title is truncated
528X listing says ‘fits 99%’
Korky gives 8-1/2–14 in. range and model-fit lookup
Treat as broad coverage, not proof of fit for a particular tank
Korky chain links versus Fluidmaster inches
Korky: 1–2 links; 400H booklet example: about 1/2 in.
Follow the installed flapper’s instructions; these are model examples
When to stop and call a licensed plumber
Supply will not shut off
Do not remove a fill-valve cap or disconnect its supply. Leave the connections in place and call a licensed plumber. The maker’s service steps require reliable isolation.
Cracked china or an external leak
Stop tightening. Isolate the supply if safe and arrange professional inspection. Fluidmaster warns that overtightening can crack the valve or tank and cause flooding.
Damaged seat or flush-valve body
A new flapper cannot correct every damaged seating surface. Have the flush assembly assessed if the isolated waterline keeps falling. Tank removal needs the exact toilet’s instructions and safe handling.
Pressure-assisted vessel
Do not open the pressure vessel or apply gravity-flapper service steps. Identify the maker and model, check its recall status and use its specified service route.
Do not add bleach or chlorine drop-in tablets to cure running. Fluidmaster’s 400H instructions warn that these in-tank cleaners can damage components and cause flooding. Follow the toilet and valve makers’ cleaning instructions.
Questions after a flapper replacement
Why is my toilet still running after I replaced the flapper?
The replacement may not fit or sit flat, its chain may hold it open, or debris may remain on the seat. A misplaced refill tube or fill valve that overfills can also keep water moving. Watch the tank refill, then isolate the supply and check whether the stored level falls.
Can a new flapper still leak?
Yes. New rubber does not establish the correct flush-opening size or tank fit. Check the mounting and chain, then clear sediment from the sealing surface. If those checks do not stop tank loss, inspect the seat and flush-valve assembly rather than assuming another identical flapper will fix it.
How much slack should the new flapper chain have?
Use that flapper’s instructions. Korky’s ghost-flushing procedure specifies 1–2 links with a nearly vertical chain. Fluidmaster’s 400H booklet gives about 1/2 inch in its flapper troubleshooting example. At rest, the chain must let the flapper seal without catching underneath it.
How do I tell a flapper leak from a fill-valve problem?
Observe the waterline. Water spilling into the overflow directs attention to fill-valve shutoff and adjustment. A falling tank level with the supply reliably closed shows stored-water loss. Check the flapper, refill-tube routing and flush assembly, as well as the outside of the tank, to locate that loss.
Why does the toilet refill every few minutes?
A working fill valve turns back on when its float drops. Leakage past a flapper or seal can lower the tank level between flushes. Fluidmaster also warns that an incorrectly inserted refill tube can siphon water. Test for tank loss before replacing a valve that may be responding normally.
Can the refill tube make the toilet keep running?
Yes. On an open-overflow installation, a tube pushed into the overflow below tank water level can cause siphoning and repeated refill cycles. Restore the prescribed clip arrangement. Canister systems may use their own refill nipple or port, so match the installed mechanism’s drawing.
What if both the flapper and fill valve are new?
Check the setup of both. Confirm flapper fit and chain slack, the fill valve’s height and waterline setting, and the refill-tube connection. Repeat the supply-off level test. Replacing both parts does not establish correct installation or repair a damaged flush-valve body.
Will a universal flapper fit my toilet?
Do not assume it will. Match the tank model and flush-opening size to the maker’s fit list. A 2-inch flapper and a 3-inch flapper target different openings. A tower or canister can need a specific seal instead of a hinged flapper.
Does a positive dye test prove the new flapper is bad?
No, EPA’s test shows colored tank water reaching the bowl after 10 minutes. That result calls for a leak-path check. Observe the overflow and inspect chain and refill routing. Flush after the test to reduce the risk of staining.
Why does the new flapper close too soon?
Check the installed flapper’s flush-duration setting and chain instructions. A replacement can seal yet be adjusted for the wrong flush. Do not tighten the chain until it holds the flapper open at rest; that trades a short flush for a continuous tank leak.
How we choose these repair parts
Why trust us
Isaac Daniels is a licensed journeyman plumber in Georgia. We use Korky’s repair procedures, Fluidmaster’s model-specific instructions and EPA’s household leak test to separate installation faults from parts that need repair.
How we pick
We match the leak path first, then check the maker’s fit range and service instructions. The Amazon offers for all three valves were new and in stock on Oct 22, 2026, 12:39 PM EDT. Each valve carries our spec score, worked out below; the diagnosis and tank fit still decide whether you need one.
We read maker documents and offer records for this comparison. The valve descriptions report documented features rather than measured differences in noise, speed or service life. Broad fit claims do not replace checking the tank model.
The spec score, check by check
Each check scores 0 to 10 and counts one fifth. It rewards a valve that sets lower for short tanks, a longer maker warranty, a bowl-refill adjustment, a documented way to clear debris and a certification named on the maker’s page. It’s our score from maker documents, not a customer rating or a noise or flow test.
Fluidmaster lists the 400H as CSA certified and code approved; Korky’s pages for the 528X and 528MP name no certification, which is why both lose points there. Korky’s lower 8-1/2 in. start and the 528X’s 10-year warranty win them points back.
Check (weight)
How it’s scored
Short tanks (⅕)
Lowest maker-stated height: 10 for 8-1/2 in.; 8.5 for 9 in.; 7 for 10 in. or more
Warranty (⅕)
Maker’s term: 10 for 10 years; 8.5 for 7 years; 7 for 5 years; 5 for under 5
Bowl refill (⅕)
10 for a bowl dial built into the valve; 9 for a separate refill adjuster in the box; 5 for none
Debris service (⅕)
10 when the maker documents a strainer or a debris flush; 5 when it doesn’t
Certification (⅕)
10 when the maker’s page names a certification; 6 when it names none
Total
Average of the checks, to one decimal
Pick
Short tanks
Warranty
Bowl refill
Debris service
Certification
Score
Fluidmaster 400H
8.5
8.5
10
10
10
9.4
Korky 528X
10
10
9
10
6
9.0
Korky 528MP
10
7
9
10
6
8.4
Selection check
Rule
Diagnosed fault
Complete fill valve only for a confirmed fill-side problem
Maker fit
Height range, float clearance and refill connection must suit the tank
Offer
Genuine model; New buy box; plain In Stock or at least 10 left
Claims
Maker facts for sizes, service and warranty; spec score from those facts only; no noise or installation-time promise
Sources
Korky, How to Fix a Running Toilet, sections on leak isolation, fill-valve shutoff and flapper identification (korky.com). Read October 20, 2026.
Korky, How to Fix Toilet Flushing on its Own, installation and chain-adjustment steps (korky.com). Read October 20, 2026.
Fluidmaster, 400H Toilet PerforMAX Fill Valve product page, adjustable height and separate water controls (fluidmaster.com). Saved maker text reviewed October 20, 2026.
Fluidmaster, 401LGR Complete Kit instructions, refill-tube positioning warning. Saved maker text reviewed October 20, 2026.
Fluidmaster, PRO45C installation instructions, refill-tube siphoning warning, step 13 (fluidmaster.com). Official indexed text checked October 20, 2026.
Korky, 2X Long Life Toilet Fill Valve, model 528X, specifications, contents and shutoff troubleshooting (korky.com). Read October 20, 2026; re-read Oct 22, 2026, 12:39 PM EDT (10-year warranty, made in USA).
Korky, QuietFILL Platinum Toilet Fill Valve, part 528MP: description, 8-1/2 to 14 inch height, contents, 5-year warranty (korky.com). Read Oct 22, 2026, 12:39 PM EDT.
U.S. EPA, WaterSense Labeled Toilets, 832-F-24-008, May 2024, Get a Handle on Leaks box. Read October 20, 2026.
Amazon Creators API, Oct 22, 2026, 12:39 PM EDT: B07N7BH94Q (400H), B08YS8C6WD (528X) and B004TKTX14 (528MP), all new and in stock; 528MP listing features read the same day.
