If your toilet tank leaks into the bowl through the flush valve itself, replace the whole valve with a kit that matches your toilet: the Fluidmaster K-507A-008 for most 2-inch toilets, the Fluidmaster K-540A-015 for 3-inch toilets. First confirm it is the valve and not just the flapper with Fluidmaster’s 30-minute water test below, and measure the flapper to tell 2-inch from 3-inch.
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Our pick for most 2-inch toilets: the Fluidmaster K-507A-008 kit, with gaskets for standard, Kohler and Gerber tanks. Check the Fluidmaster K-507A-008 on Amazon
First, Make Sure It Is the Flush Valve
Dye tablets tell you the tank leaks, but not where. Fluidmaster’s water test finds the leak point:
- With the tank full, turn off the water at the stop valve. Do not flush.
- Mark the water level inside the tank.
- Wait 30 minutes. If the level dropped, mark the new level and keep waiting until it stops dropping.
The water stops at the height of the leak:
- Stops at the lip of the flapper: a worn flapper or a damaged seat under it. Start with a new flapper (our flapper replacement guide). If the seat is rough, a seat repair kit such as the Fluidmaster 555C below (sold on Amazon as the 555CRP8) fixes it without pulling the tank.
- Drains to the bottom of the tank: the gasket that seals the flush valve to the tank is worn. Fluidmaster’s fix is a new flush valve. This is what the kits on this page are for.
- Drops 1/4 to 1/2 inch and stops: the refill tube may be pushed down into the overflow pipe, or the overflow pipe is cracked. Clip the refill tube above the overflow; a cracked overflow means a new flush valve.
Toilet still running after all that? It may be the fill valve instead; see flapper vs fill valve and toilet still running after replacing the flapper.
2-Inch or 3-Inch?
Most toilets have a 2-inch or a 3-inch flush valve, set by the size of the hole the maker bored in the tank. Fluidmaster’s way to tell: measure the old flapper across. About 3 inches across means a 2-inch valve; about 4 inches across means a 3-inch valve. More detail in 2-inch vs 3-inch toilet flapper. If your Kohler has a tall canister in the middle instead of a flapper, it is a different design; see Kohler canister flush valve seal.
Flush Valve Kits Compared
The three full flush valve kits are for two-piece toilets, where the tank bolts onto the bowl.
| Pick | Kit | Size | Tank comes off? |
|---|---|---|---|
| Best 2-Inch Overall | Fluidmaster K-507A-008 2-Inch Flush Valve Kit | 2-inch | Yes |
| Best 2-Inch Alternative | Korky 4030 2-Inch Flush Valve and Gasket Kit | 2-inch | Yes |
| Best 3-Inch | Fluidmaster K-540A-015 3-Inch Flush Valve Kit | 3-inch | Yes |
| No Tank Removal | Fluidmaster 555CRP8 2-Inch Flush Valve Repair Kit | 2-inch | No |
Every kit was sold and shipped by Amazon, new and in stock, when we checked on September 24, 2026.
Fluidmaster K-507A-008 Universal 2-Inch Flush Valve Kit
2-inch · adjustable-height overflow tube · PerforMAX adjustable flapper · universal tank-to-bowl gasket system · tool and hardware included · 5-yr warranty
Fluidmaster’s own leak-test page names this kit as its fix for a cracked overflow pipe. It replaces the whole valve, and the overflow tube twists to adjust its height for your tank. Fluidmaster says it fits most 2-inch flush valve toilets and is best for 1.28 and 1.6 gallon-per-flush toilets, and can be used on older 3.5 GPF models.
The part that makes it universal is the tank-to-bowl gasket system: Fluidmaster says to use the red gasket for standard designs, the black triangle gasket for Kohler models, and both together for Gerber. The listing names the flush valve, gasket, tool and hardware as included.
- Named by Fluidmaster’s leak-test page as the fix for a cracked overflow pipe
- Red (standard) and black triangle (Kohler) tank-to-bowl gaskets included; used together, they fit Gerber
- Overflow tube adjusts with a twist; tool and hardware included
- The tank has to come off the bowl to install it
- 2-inch only; a 3-inch toilet needs the K-540A-015
- Fluidmaster warns that bleach or chlorine in-tank cleaners damage its parts
Verdict: The first kit to try on a standard two-piece, 2-inch toilet. It covers the valve, flapper and tank-to-bowl seal in one repair.
Korky 4030 Adjustable 2-Inch Flush Valve and Gasket Kit
2-inch · height adjusts 7 to 11-1/2 inches · flush valve, tank-to-bowl gasket, flapper and hardware · 1.28 to 5 GPF · made in US
The Korky 4030 is the alternative when you want the valve height range spelled out. The listing says the flush valve adjusts from 7 to 11-1/2 inches with no tools or hacksaw, and that the kit includes the flush valve, tank-to-bowl gasket, flapper, hardware and instructions.
The listing rates it for all standard 2-inch toilets from 1.28 to 5 gallons per flush, with an oversized tube and funnel that it says reduce flood risk, and red Chlorazone rubber that it says holds up to treated city water, hard water and well water.
- Adjusts from 7 to 11-1/2 inches with no cutting, per the listing
- Flush valve, tank-to-bowl gasket, flapper and hardware included
- Listed for 1.28 through 5 GPF 2-inch toilets
- The tank has to come off
- 2-inch only
- No Kohler-specific gasket named in the listing; for a Kohler 3-bolt tank, Fluidmaster recommends Kohler’s own gasket and bolt kit
Verdict: A solid second choice for a 2-inch two-piece toilet, especially a tall tank where you want the height range stated.
Fluidmaster K-540A-015 Universal 3-Inch Flush Valve Kit
3-inch · adjustable height · adjustable flapper · 3-inch gasket · silicone seal · tool and hardware included · 5-yr warranty
Newer high-efficiency toilets often use a 3-inch flush valve. Fluidmaster says the K-540A-015 replaces most 3-inch flush valves, including Toto and American Standard, and fits 3-inch HET toilets built 2005 and newer. If only the flapper or seal is leaking, the maker-specific parts are in our TOTO flapper guide and American Standard flapper guide.
Fluidmaster lists the 3-inch flush valve, gasket, hardware and a toilet tool as included, with an adjustable overflow tube and an adjustable flapper to set how much water each flush uses.
- Replaces most 3-inch flush valves, including Toto and American Standard, per Fluidmaster
- Gasket, hardware and tool included
- Adjustable overflow height and flapper
- The tank has to come off
- Measure first: a 2-inch toilet needs a 2-inch kit
- Not for canister-style flush valves; see the Kohler canister note above
Verdict: The kit for a 3-inch high-efficiency toilet whose tank leaks into the bowl through the valve or its gasket.
Fluidmaster 555CRP8 PerforMAX 2-Inch Flush Valve Repair Kit
2-inch · peel-and-stick seat over the old valve · PerforMAX adjustable flapper · 1.28 to 3.5 GPF
If the flapper keeps leaking because the seat it closes on is rough or pitted, this kit puts a new seat on top of the old one without taking the tank off. Fluidmaster calls it the quickest way to fix leaks from the flush valve: peel the paper off the sealant ring and stick it onto the existing 2-inch flush valve seat. A PerforMAX adjustable flapper comes with it.
It fixes the seat, not the gasket under the tank. If Fluidmaster’s water test drains to the bottom of the tank, you need a full flush valve instead. One more thing from the listing: it is marked as a closeout item with a limit of one per order, and it says returns will not be honored.
- No tank removal, per Fluidmaster
- New seat plus an adjustable flapper in one kit
- Listed for 1.28 to 3.5 gallon-per-flush 2-inch toilets
- Does not fix a leaking tank-to-bowl gasket or a cracked overflow tube
- 2-inch only
- The listing marks it a closeout with a limit of one and no returns
Verdict: Worth it when the water test stops at the flapper lip and a new flapper alone did not stop the leak.
I send an occasional note when a new guide goes up — repairs worth doing yourself, and the parts that actually last. No spam, no course funnel; written by a licensed Georgia plumber.
Getting the New Valve to Seal
- Do not overtighten the lock nut. Fluidmaster says the nut that holds the flush valve to the tank should go no more than 1/2 turn past hand tight. A new valve that leaks at its gasket may have a loose nut; snug it within that limit.
- Get the tank bolt washers in the right order. Fluidmaster’s order, from inside the tank down: bolt head, rubber washer, tank, then (only if there is a gap between tank and bowl) a metal washer and thin hex nut, then the bowl, rubber washer, metal washer and hex nut. Never a metal washer right under the bolt head inside the tank; Fluidmaster says that causes a leak.
- Kohler 3-bolt tanks: Fluidmaster recommends Kohler’s triangular tank-to-bowl gasket and bolt kit.
- Skip bleach tablets in the tank. Fluidmaster warns that in-tank drop-in cleaners with bleach or chlorine damage tank parts and may cause flooding.
Parts You’ll Also Need
Pulling the tank is the time to replace two small parts that fail on their own. Each was sold or shipped by Amazon, new and in stock, when we checked on September 24, 2026; details come from each Amazon listing.
- Tank-to-bowl bolts, if yours are rusted or the leak is at the bolts. The Fluidmaster 6101 tank-to-bowl bolt kit has solid-brass bolts with washers and nuts and is listed as fitting all brands. Not for Kohler 3-bolt tanks (see above).
- A new supply connector. You disconnect it to take the tank off anyway, so it is a good time to swap in a Fluidmaster B1T12 braided stainless toilet connector. Check which size your toilet takes in our toilet supply line size guide.
FAQ
How do I know if I need a new flush valve or just a flapper?
Use Fluidmaster’s water test: turn off the water with the tank full, mark the water level and wait. If it drops to the lip of the flapper, the flapper or its seat is leaking. If it drains to the bottom of the tank, the flush valve gasket is worn and the flush valve needs replacing. If it drops 1/4 to 1/2 inch and stops, check the refill tube and the overflow pipe.
Is my toilet flush valve 2-inch or 3-inch?
Fluidmaster’s method is to measure the flapper across. A flapper about 3 inches across goes on a 2-inch flush valve; one about 4 inches across goes on a 3-inch valve.
Can you replace a toilet flush valve without removing the tank?
Not the whole valve. The flush valve is held to the tank by a nut under it, so a full replacement means taking the tank off the bowl. Fluidmaster’s 555C kit is the exception for one problem: it sticks a new seat onto the old 2-inch valve without tank removal, which fixes a pitted seat but not a leaking gasket.
How tight should the flush valve nut be?
Fluidmaster says the flush valve lock nut should be tightened no more than 1/2 turn beyond hand tight. A new valve that leaks at the gasket may have a loose lock nut.
Why does my toilet tank leak at the bolts?
One common cause is washers in the wrong order. Fluidmaster says never to put a metal washer directly under the bolt head inside the tank because it will cause a leak; the rubber washer goes there. For Kohler 3-bolt tanks it recommends Kohler’s own triangular gasket and bolt kit.
Why Trust Plumbing Picks
Plumbing Picks is written by a licensed Georgia plumber — journeyman license #JP507711 — and the license is verifiable through the Georgia Secretary of State’s license verification portal. The steps on this page follow the manufacturer sources listed below, and product details come from the Amazon listing or the maker, not from hands-on testing. Nobody pays to be included here. Where a link goes to Amazon we may earn a commission at no extra cost to you. More on who writes this and how we pick.
Sources
- Fluidmaster: Testing the toilet tank water for a leak into the toilet bowl
- Fluidmaster: Determining whether you have a 2″ or 3″ flush valve and flapper
- Fluidmaster: K-507A-008 Universal 2-In. Adjustable Toilet Flush Valve Repair Kit
- Fluidmaster: K-540A-015 Universal 3-In. Flush Valve Repair Kit
- Fluidmaster: 555C 2″ Universal Flush Valve Repair Kit With Flapper
- Fluidmaster: Installing / resolving leaks at the tank bolts
Bottom Line
Run Fluidmaster’s water test first. If the water drains to the bottom of the tank or the overflow pipe is cracked, replace the flush valve: the Fluidmaster K-507A-008 for most 2-inch toilets, the Korky 4030 if you want the height range stated, and the Fluidmaster K-540A-015 for 3-inch toilets. If it stops at the flapper lip and a new flapper did not fix it, the 555C seat kit repairs the seat without pulling the tank.