Toilet keeps running after the tank fills
The fill valve shuts off briefly, then starts again, or water keeps trickling into the bowl.
Start here: See if the flapper is sitting flat and make sure the tank water is not running into the overflow tube.
Toilet tank repair
Toilet Flush Valve Repair
Direct answer: If the toilet is running, ghost flushing, or barely flushing, take the lid off and watch what the parts are doing before you buy anything. A lot of these repairs are just a flapper, seal, chain adjustment, or water-level fix. Replace the full flush valve when the overflow tube is cracked, the seat is damaged, the valve body is loose, or water is leaking at the large tank opening.
Most likely: Most of the time, the trouble is a worn flapper, a tired seal, chain slack, water set too high or too low, or an old flush valve that no longer gives the flapper a clean place to land.
This is a tank-lid-off repair. Watch one flush and one refill. You will usually see the problem: the flapper does not lift, it will not sit flat, water is spilling into the overflow tube, or the valve body is loose. Fix that part, then test it long enough to make sure you did not create a new leak.
Don’t start with: Do not start by replacing the whole toilet or grabbing the first universal kit on the shelf. Look at the flapper, valve seat, overflow tube, tank gasket, tank bolts, and fill valve first.
Toilet runs or refills by itself?Look at the flapper, valve seat, and water level before you take the tank apart.
Weak flush or short flush?Check the chain, flapper lift, and tank water level before replacing the valve body.
Last reviewed: 2026-05-22
What kind of flush valve problem do you have?
Toilet ghost flushes or refills randomly
The toilet is quiet for a while, then refills even though nobody flushed.
Start here: Add dye to the tank and wait. If color shows up in the bowl, water is sneaking past the flapper, seal, or valve seat.
Flush is weak, short, or needs the handle held down
The bowl starts to flush but quits early, or the flapper drops before enough water leaves the tank.
Start here: Check chain slack, tank water level, and whether the flapper is the right style for that valve.
Valve is damaged or leaking under the tank
The overflow tube is cracked, the valve body is loose, or water drips from the center underside of the tank.
Start here: Dry the tank bottom and find where water appears first. If the valve body is damaged, plan on replacing the valve or gasket.
Most likely causes
1. Worn toilet flapper or flush valve seal
The flapper or seal is the part that closes against the flush valve. When it warps, hardens, or gets coated with mineral buildup, tank water leaks into the bowl.
Quick check: Put a few drops of food coloring in the tank and wait without flushing. If color appears in the bowl, water is leaking past the flush valve area.
2. Chain, handle, or flapper lift problem
Too much slack gives a weak lift. Too little slack can hold the flapper open. Either way, the toilet may run, flush weakly, or need the handle held down.
Quick check: Flush with the lid off and watch whether the flapper lifts fully, drops cleanly, and has a little slack when closed.
3. Damaged flush valve seat or cracked overflow tube
A new flapper will not seal well on a rough, cracked, warped, or broken flush valve seat. A cracked overflow tube can leak tank water even when the flapper is fine.
Quick check: Wipe the seat clean and inspect it with a flashlight. Look for grooves, chips, cracks, a broken tube, or a valve body that moves in the tank.
4. Flush valve gasket or tank-to-bowl seal leak
If water leaks outside the toilet near the large center tank opening, the flush valve gasket, locknut, or tank-to-bowl gasket may not be sealing.
Quick check: Dry the underside of the tank, wrap tissue around the center opening and tank bolts, then refill and flush once to see what gets wet first.
Step-by-step fix
Step 1: Watch one full flush and refill with the tank lid off
You can usually spot the problem from inside the tank faster than you can guess from the outside.
- Remove the tank lid and set it on a towel where it cannot slide or crack.
- Flush once while watching the handle arm, chain, flapper, flush valve seat, overflow tube, and water level.
- See whether the flapper lifts all the way, drops flat, and stays closed after the flush.
- Make sure the fill valve shuts off with the water below the overflow tube.
- Listen after the tank fills. If it gets quiet and then refills later by itself, water is probably leaking into the bowl.
Next move: You should know what to try first: chain adjustment, water-level adjustment, a flapper or seal, or a closer look at the full valve. If the symptom only happens occasionally, mark the tank water level, wait 15 to 30 minutes, and see whether the water level drops without flushing.
What to conclude: Water leaking into the bowl usually means flapper, seal, or valve seat. Water going into the overflow tube is a fill valve problem. Weak lift is usually handle or chain.
Stop if:
- The shutoff valve leaks or will not close if you need to drain the tank.
- The tank, bowl, or overflow tube is visibly cracked.
- Water is already leaking onto the floor or ceiling below.
Step 2: Figure out whether you need a flapper, seal, or full valve
A new flapper only helps if it has a smooth valve seat to close against. If the plastic valve body is damaged, the cheap part will not hold for long.
- Clean the flush valve seat with a cloth and check for mineral buildup, rough spots, chips, or a groove where the flapper lands.
- Check the flapper or seal for warping, cracks, swelling, or a soft edge that no longer sits flat.
- If the flush valve uses a tower or canister seal, inspect the seal style and confirm whether the valve body itself is still solid.
- Gently touch the overflow tube and valve body. They should not wobble, split, or flex at the tank opening.
- Use a flapper or seal when the valve body looks good. Replace the full flush valve when the seat is damaged, the tube is cracked, the valve is loose, or the tank gasket is leaking.
Next move: You can keep the repair small when the valve is still sound, and replace the valve when the plastic itself is the problem. If the seat is rough or the plastic feels brittle, replace the full flush valve. A new rubber part needs a solid surface to seal against.
What to conclude: Start with the sealing surface, then the lift, then the tank connection. That order keeps you from chasing the wrong part.
Step 3: Fix chain slack and water level before buying parts
Plenty of weak flushes come from a simple adjustment, not a broken valve.
- Set the chain with a little slack when the flapper is closed. It should not be tight enough to lift the flapper at rest.
- Flush and confirm the flapper rises high enough for a full flush without binding against the overflow tube or tank wall.
- If holding the handle down gives a better flush, adjust chain length or use the correct flapper style for the valve.
- Check the tank water level. It should be near the marked fill line or at the normal level for the tank, while still below the overflow tube.
- If water is entering the overflow tube after the tank fills, adjust or repair the fill valve before blaming the flush valve.
Next move: A normal flush with a stable water level means the flush valve body may not need replacement. If the toilet still leaks into the bowl or the flapper cannot seal on the valve seat, replace the worn sealing part or the valve body.
Step 4: Make the repair that matches what you found
Once you know what is leaking or sticking, the fix is usually straightforward: rubber part, valve body, or tank gasket.
- Replace the toilet flapper if the valve seat is smooth and the flapper is warped, swollen, or closing poorly.
- Replace the toilet flush valve seal if your toilet uses a tower or canister-style seal and the valve body is otherwise sound.
- Replace the toilet flush valve assembly if the seat is cracked, rough, warped, the overflow tube is damaged, or the valve body moves in the tank.
- If water leaks outside at the center tank opening, replace the flush valve gasket or full assembly and consider fresh tank-to-bowl hardware while the tank is off.
- Match the replacement by flush valve size, overflow tube height, flapper or canister style, and tank layout.
Repair guide: How to Replace a Toilet Flapper
Related repair guide: How to Replace a Toilet Flush Valve Seal
Step 5: Test it long enough to catch slow leaks
Toilet tank repairs can look fine on the first flush and leak later. Take a few extra minutes now.
- Let the tank fill fully and confirm the water stops below the overflow tube.
- Flush at least three times and watch the flapper or tower close each time.
- Dry the underside of the tank and check the center opening, tank bolts, and supply connection for drips.
- Mark the tank water level and wait 15 to 30 minutes without flushing.
- If the water level drops or the toilet refills by itself, repeat the dye test and inspect the flush valve seat again.
A good result: A steady tank level, dry outside, and strong repeated flushes mean the repair is holding.
If not: If the tank still loses water, do not keep swapping random parts. The remaining issue is usually fit, seat damage, chain tension, fill level, or a hidden crack.
What to conclude: The job is done when the original problem is gone and the outside of the tank stays dry.
Replacement Parts
Toilet flapper on Amazon
Toilet flush valve seal on Amazon
Toilet flush valve assembly on Amazon
Toilet tank bolt set with washers on Amazon
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FAQ
What is the difference between a toilet flapper and a flush valve?
The flush valve is the larger piece mounted in the tank opening. The flapper or seal is the rubber piece that closes against it after each flush.
How do I know if I need a new flush valve instead of a flapper?
If the valve seat is smooth and the flapper is worn, replace the flapper. If the seat is cracked, rough, warped, the overflow tube is broken, or the valve body is loose, replace the flush valve.
Can a bad flush valve cause a toilet to run?
Yes. If water leaks past the flush valve seat, flapper, or seal, the tank level drops and the fill valve turns back on to refill it.
Can a flush valve cause a weak flush?
Yes, but not always because it is broken. A flapper that closes too soon, the wrong flapper style, too much chain slack, or low tank water can all create a weak flush.
Do I have to remove the tank to replace a toilet flush valve?
Usually yes. The flush valve locknut and tank-to-bowl gasket are under the tank, so a full flush valve replacement normally requires removing the tank from the bowl.
Why is water going into the overflow tube?
That is usually a fill valve or water-level problem, not a flush valve leak. Adjust or repair the fill valve so the tank stops filling below the top of the overflow tube.