Toilet keeps running: separate flapper leaks from fill-valve problems

Open an ordinary gravity tank and find the shutoff

You need a flashlight, towel, phone camera, food coloring and a timer. Set a loose ceramic lid flat on the towel and photograph the tank. These steps cover a water-filled gravity tank with a float-operated fill valve. A sealed pressure vessel, electronic toilet or concealed tank needs a different procedure; leave its assembly closed.

Find the water shutoff beside the toilet. Close it and verify that incoming water stops before changing a hose, chain or seal. A seized or leaking shutoff needs repair first. If water is on the floor or the bowl is rising, stop the checks and isolate the water at a safely accessible shutoff.

Watch where the water goes

If the bowl drains normally and nothing is leaking outside, flush once and watch the whole refill. The fill valve brings in water; the flapper or canister seal holds that water in the tank. The small refill hose supplies the bowl during filling. Its normal stream into the overflow opening is different from the tank water rising over that opening.

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For intermittent refilling, start with the chain and hose

With the supply off and handle at rest, look at the chain from lever to flapper. If it is taut, unclip it from the lever and reconnect with a little slack so the seal rests flat. On the chain-operated Kohler mechanisms in the linked guide, the allowance is one or two links. Keep the loose tail from slipping beneath the seal. If the handle itself sticks, check that it returns before adjusting the chain again. Push-button rods and cables use a different adjustment.

Check the thin refill hose before blaming a new flapper. On a conventional open-overflow arrangement, its end belongs in the manufacturer’s clip at the overflow opening, not pushed down below the tank waterline. A submerged end can siphon water from the tank. On Kohler canister models, the hose instead connects to the designated port at the top of the flush valve. Restore that connection using the correct fitting; do not reroute every canister hose into an open tube.

After correcting an obvious snag or hose displacement with the supply off, restore the supply and observe another complete flush/refill. If the tank still loses water, use the dye check below.

Use dye to confirm a leak, not to name the part

Use this check only on a conventional water-filled tank after filling has stopped, the water is below the overflow opening and the hose is correctly positioned. If the tank is continuously overflowing, follow the next section first: dye entering the bowl along that route would not prove a bad flapper. Start with clear bowl water so a color change is visible.

1. Put a few drops of food coloring in the tank water. Do not put it in the bowl or operate the flush handle.
2. Wait 10 minutes without using the toilet, then inspect the bowl for color. EPA WaterSense uses this interval to check for toilet leaks.
3. Flush immediately after the check to clear the dye and help avoid staining. Do not leave it overnight.

• Color in the bowl confirms tank water is escaping into it. With overflow and hose siphoning excluded, inspect the flapper/canister seal and its seat. The test alone cannot distinguish a worn seal from debris, a damaged seat or a flush-valve-base leak.
• No color means no leak was detected during that short test. If the toilet normally refills only hours later, keep listening through its usual recurrence period after flushing the dye away.

For constant overflow, check the fill control

Locate the toilet’s specified waterline, not an old mineral stain. Watch whether the float moves freely or catches on the tank wall, hose or linkage. Correct only an obvious displaced part using its installation instructions. Do not bend an unidentified arm or cut the overflow tube to make a level setting work.

If the float moves normally but filling stops too high, use the installed valve’s water-level adjustment and the toilet’s target mark. For example, Kohler guide 1004604-2-G, covering models including K-3422 and K-3423, specifies a threaded rod: counterclockwise lowers the level and clockwise raises it. This is a named-system example, not a direction that applies to every replacement valve.

After an adjustment, flush and let the tank refill to see the new shutoff level. Lowering an adjustment does not by itself remove water already in the tank. The result should reach the specified mark, stop below the overflow and stay quiet.

If filling continues despite the correct setting and unobstructed movement, sediment inside the fill valve or a worn valve is a likely next check. Find the cleaning procedure for that exact valve, or have it serviced/replaced. Do not twist off an unidentified valve cap with the water on. Kohler specifies different procedures for its two-piece and low-profile one-piece toilets.

Replace a worn flapper without changing the working fill valve

First match the toilet model to its replacement part. A hinged rubber flapper, a canister’s bottom seal and a dual-flush assembly are different parts. Bring the model photo and old-part photo to the lookup instead of choosing by color. The following is the ear-mounted flapper procedure shown by Korky.

1. Close the supply and flush to lower the tank water. Verify that it stays off. Photograph the lever attachment before disconnecting the chain.
2. Release the flapper’s ears from their mounting tabs. Clean loose sediment from the seat with a soft cloth; a nicked or cracked seat will not be repaired by a new rubber part.
3. Fit the compatible flapper onto both tabs without twisting it. Attach the chain to the lever with the specified small amount of slack. Check that it lifts freely, drops onto the seat and does not trap its own chain.
4. Open the supply and let the tank fill. Check several complete flushes, then repeat the dye test. A quiet tank with the supply still closed is not a completed repair.

• For a canister, replace the matched bottom seal by that valve’s release procedure; do not pry the whole tower out as if it were a hinged flapper.
• If the leak returns with the correct seal seated flat, check the seat and valve-base gasket. Buying another fill valve will not close a leak beneath the flush valve.

Three running toilets, three different repairs

These are illustrative situations, not customer jobs or measured failure rates. The useful comparison is what changed after one correction.

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Prove it works with the water back on

An empty tank or a quiet toilet with its supply closed proves nothing about the repair. Finish reassembly, open the supply normally and test actual operation. After any part replacement, check again over the next several days for fresh leakage.

• Flush several times, allowing each refill to finish. The handle returns, the seal closes and the tank stops filling at its specified level without spilling over the overflow.
• Fit the lid correctly and test again. On dual-flush models, try both buttons and allow a complete refill between flushes.
• After the last fill, repeat the prepared 10-minute dye check if tank-to-bowl leakage was the symptom; flush the dye away afterward.
• Listen later during the interval when refilling used to recur. A short quiet spell does not establish that an occasional leak is gone.
• Check the shutoff, supply connections, tank underside and floor for new moisture. If any are wet, stop using the toilet and address that leak.