How to Fix a Toilet Flush Valve Leak: Step-by-Step Guide

Water leaking from the toilet tank into the bowl is one of the top water-wasting household plumbing problems. After a new flapper fails to stop the running, the flush valve itself is usually the culprit.

Water leaking from the toilet tank into the bowl is one of the top water-wasting household plumbing problems. After a new flapper fails to stop the running, the flush valve itself is usually the culprit. This guide walks through diagnosing the failure, replacing the flush valve and tank-to-bowl gasket, and confirming a proper seal — all without calling a plumber.

What You Need

• Universal toilet flush valve replacement kit — fits most standard two-piece toilets; $10–$30
• Tank-to-bowl gasket (spud washer) — replace it any time you remove the tank; $5–$10
• Toilet tank bolt set with gaskets — brass bolts corrode; replace them while the tank is off; $6–$10
• Adjustable wrench or channel-lock pliers
• Flathead screwdriver
• Hacksaw (in case old bolts are corroded and need to be cut)
• Sponge and bucket
• Rubber gloves
• Putty knife

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Step 1: Confirm the Flush Valve Is the Problem

Before removing the tank, confirm this is a flush valve issue and not something simpler.

The dye test:

Drop five drops of food coloring or a dye tablet into the toilet tank. Do not flush. Wait 15 minutes. If color appears in the bowl, water is leaking past the flush valve assembly. A few drops of color is a small leak (likely flapper); a strong color transfer indicates a significant flush valve failure.

Inspect the flapper first:

If the dye test is positive and the flapper is less than three years old, remove it and run your finger around the flush valve seat — the flat circular ring at the bottom of the tank that the flapper presses against. If it feels smooth and uniform, install a new flapper (the flapper, not the whole valve, is the fix). If you feel roughness, chips, pitting, or mineral deposits you cannot wipe away, the seat is damaged and the full flush valve must be replaced.

Check for tank-to-bowl leaks:

Dry the outside of the tank and the tank-to-bowl connection with a paper towel. Flush the toilet and watch the connection point. A drip at the underside of the tank or where the tank meets the bowl indicates a failed spud washer or corroded tank bolt gaskets — this repair is also part of flush valve replacement.

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Step 2: Turn Off the Water and Empty the Tank

1. Turn the water supply shut-off valve clockwise until it stops. The valve is on the wall behind the toilet near the floor.
2. Flush the toilet to drain the tank. Hold the handle down for a complete drain.
3. Use a sponge to remove all remaining water from the tank. Wring it into a bucket. Get every drop — you will be removing the tank and do not want water everywhere.
4. Disconnect the water supply line from the bottom of the tank by turning the coupling nut counterclockwise. Catch any drips with a towel.

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Step 3: Remove the Tank

Two-piece toilets (separate tank and bowl) require tank removal to access the flush valve. One-piece toilets have the tank and bowl integrated — if you have a one-piece toilet, consult the manufacturer for flush valve access, as it varies by design.

To remove the tank:

1. Inside the tank, locate the two or three tank bolts. They run through the tank bottom and thread into nuts underneath.
2. From under the tank, use an adjustable wrench to hold one nut while you use a flathead screwdriver to hold the bolt head still inside the tank. Turn the nut counterclockwise.
3. If the bolts spin freely (they have corroded at the nut), slide a hacksaw blade under the tank and cut the bolts — a common occurrence on tanks over ten years old.
4. Once all nuts are removed, lift the tank straight up off the bowl. Set it upside down on a folded towel on the floor.
5. Remove the large rubber spud washer that was between the tank and bowl — it slides over the flush valve shank. Set it aside; you will replace it with a new one.

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Step 4: Remove the Old Flush Valve

With the tank upside down on the floor:

1. Locate the large plastic locknut that secures the flush valve shank to the tank bottom. It is a large nut (2 to 3 inches across) threaded onto the outside of the flush valve shank, viewed from the tank bottom (which is now facing up since the tank is inverted).
2. Use channel-lock pliers or a large adjustable wrench to grip the locknut and turn it clockwise (remember — the tank is upside down, so directions are reversed). Unthread the locknut completely.
3. Reach inside the tank (now accessible since you are looking at the bottom) and push the flush valve shank upward and out through the tank hole.
4. The old flush valve lifts free from the tank. Inspect the hole for cracks or damage before proceeding.
5. Scrape any old rubber residue from around the tank hole with a putty knife and wipe it clean.

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Step 5: Install the New Flush Valve

Universal flush valve kits (such as the Fluidmaster 507AK or equivalent) fit most standard toilet tanks. Read the kit instructions as specific installation details vary slightly.

General installation:

1. Insert the new flush valve shank down through the tank hole from the inside. The overflow tube should be oriented correctly — on most toilets it faces toward the back of the tank.
2. Check that the overflow tube height is correct: it should sit approximately 1 inch below the top rim of the tank and at least 1 inch above the water level you intend to set.
3. From outside the tank bottom, thread the new locknut onto the flush valve shank by hand. Tighten with pliers — firm and snug, not overtorqued. The rubber gasket on the shank does the sealing; excessive tightening can crack the tank.
4. Press the new flapper (included in most kits) onto the overflow tube ears and hook the chain to the flush handle arm. Leave about 1/2 inch of slack in the chain.

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Step 6: Install New Tank-to-Bowl Gasket and Bolts

With the tank still inverted and the new flush valve installed:

1. Slide the new spud washer (tank-to-bowl gasket) over the flush valve shank from outside the tank. Push it up firmly against the tank bottom — it should seat flat against the porcelain.
2. Insert the new tank bolts through the bolt holes in the tank bottom from inside the tank. Each bolt should have a rubber washer on the inside face (between the bolt head and the tank interior surface).
3. From outside the tank, thread a metal washer and nut onto each bolt just finger-tight — enough to hold them in position while you flip the tank over and set it on the bowl.

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Step 7: Remount the Tank

1. Flip the tank upright and lower it onto the bowl, aligning the bolt holes in the tank with the bolt holes in the bowl.
2. The spud washer must center over the flush valve opening in the bowl. Press the tank down gently to seat the gasket.
3. From underneath the bowl, thread the tank bolt nuts on: hand-tighten first, then use a wrench. Alternate sides (tighten the left bolt, then the right bolt) to seat the tank evenly. Snug the bolts until the tank is secure and does not wobble — do not overtighten. Porcelain cracks with excessive force.
4. Check that the tank is level and sits flush against the bowl with no gaps at the gasket.

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Step 8: Reconnect the Water and Test

1. Reconnect the supply line to the fill valve at the bottom of the tank. Hand-tighten, then snug with pliers.
2. Open the shut-off valve slowly. Allow the tank to fill completely.
3. Check the water level — it should sit 1 inch below the top of the overflow tube. Adjust the fill valve float if needed.
4. Flush the toilet and watch for drips at the tank-to-bowl connection underneath. Dry the connection first with paper towels so any new drip is immediately visible.
5. Repeat the dye test: add food coloring to the tank, wait 15 minutes without flushing, and confirm no color transfers to the bowl.

If no drips are visible and the dye test is negative, the repair is complete.

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When to Replace the Entire Toilet Instead

If the tank has hairline cracks, the porcelain is chipped around the bolt holes, or this is the third repair in five years on the same toilet, replacing the entire toilet is a better long-term investment. A basic WaterSense 1.28 GPF toilet costs $100 to $200 and uses significantly less water than toilets manufactured before 1994.

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Related Reading

• How to Replace a Toilet Flapper
• How to Fix a Running Toilet
• How to Install a New Toilet