What You Need to Know Before Starting

The flush valve seal is the rubber or silicone ring at the bottom of your toilet tank that lets water flow into the bowl when you flush. When it wears out, your toilet runs constantly or won't flush with full force. Replacing it is a straightforward repair you can do yourself in about 30 minutes with basic tools — no plumber needed.

Before you begin, turn off the water supply at the shutoff valve behind or below your toilet. Flush once to empty the tank, then turn the shutoff valve clockwise until it stops. If you cannot find the shutoff valve or it does not work, you can still do this repair, but you will need to be more careful about water spilling.

Key Takeaways

  • The flush valve seal sits inside the tank at the base of the overflow tube and costs between $8 and $25 depending on your Kohler model.
  • You will need a large adjustable wrench or a flush valve socket, a screwdriver, and a bucket to catch water that remains in the tank.
  • The entire job takes about 30 minutes and requires draining the tank, unbolting the flush valve from underneath, and sliding out the old seal.
  • Kohler flush valve seals come in different sizes, so check your toilet model number on the tank before ordering a replacement.

Gather Your Tools and the Right Replacement Seal

You will need an adjustable wrench (or a flush valve socket if you have one), a flathead screwdriver, a bucket, and old towels or rags. Have your phone or a pen and paper ready to write down your toilet model number.

Find the model number on the back of your toilet tank, usually stamped or printed near the water line. It starts with a letter or two followed by numbers — for example, K-3977 or K-4636. Write this down and search for "Kohler [model number] flush valve seal" online or call a plumbing supply store. The seal itself is inexpensive (usually $8 to $25), but ordering the wrong size wastes time. Some Kohler models use a standard 2-inch seal, while others use 2.5-inch or 3-inch seals.

Drain the Tank and Remove the Fill Valve

Turn off the water at the shutoff valve and flush to empty the tank. Use a sponge or towel to soak up any remaining water in the tank bottom, then place a bucket underneath the tank to catch drips.

Look inside the tank. You will see the fill valve (the tall assembly on the left side that refills the tank) and the flush valve (the larger assembly in the center or back). Disconnect the water supply line from the fill valve by turning the nut counterclockwise by hand or with a wrench. Some water will drip — that is normal. You do not need to remove the fill valve itself, just disconnect the water line so it does not get in your way.

Unbolt the Flush Valve from Below the Tank

Get under the toilet and look at the bottom of the tank. You will see a large nut holding the flush valve in place — this is called the flush valve locknut. It is usually brass or stainless steel and about 1.5 inches across. Use your adjustable wrench or flush valve socket to turn this nut counterclockwise. It will be tight, so explore steady pressure rather than jerking.

As you loosen the nut, water may drip from the tank. Keep your bucket underneath. Once the nut is loose enough to turn by hand, unscrew it the rest of the way and set it aside. You may also see a rubber washer under the nut — keep this; you will reinstall it.

Lift Out the Flush Valve and Replace the Seal

From inside the tank, grasp the flush valve assembly and pull it straight up. It should come free without much force. If it is stuck, wiggle it gently side to side. Set it on a towel so water does not spill on your floor.

Look at the bottom of the flush valve. You will see a rubber or silicone ring — this is the seal you are replacing. It may be cracked, flattened, or discolored. Pry it out gently with a flathead screwdriver. Do not scratch the metal surface underneath; if you do, the new seal will not sit flush and your toilet will still run.

Rinse the metal surface with clean water and dry it completely with a towel. Slide the new seal onto the bottom of the flush valve, making sure it sits evenly all the way around. The seal should fit snugly but not require force.

Reinstall the Flush Valve and Reconnect Water

Lower the flush valve back into the tank opening, aligning it so the overflow tube (the hollow pipe in the center) lines up with the fill valve. From underneath, thread the locknut back onto the flush valve by hand first, then tighten it with your wrench. Turn it clockwise until it is snug — do not over-tighten, as this can crack the tank. You should feel resistance; stop when you do.

Reconnect the water supply line to the fill valve by turning the nut clockwise by hand, then tighten it slightly with a wrench. Turn the shutoff valve counterclockwise to restore water to the tank. Let the tank fill completely, then flush several times to test. The toilet should flush with normal force and the tank should refill without running constantly.

Troubleshooting If the Toilet Still Runs

If water still runs into the bowl after you flush, the seal may not be sitting evenly. Turn off the water, drain the tank again, and remove the flush valve once more. Check that the metal surface is clean and dry, and that the new seal is centered and not twisted. Reinstall and test again.

If the toilet leaks from underneath the tank, the locknut may not be tight enough, or the rubber washer under the nut may be damaged. Turn off the water and tighten the locknut a quarter turn at a time, testing between turns. If tightening does not stop the leak, the washer may need replacing — ask your plumbing supply store for a flush valve locknut washer kit for your model.

Frequently Asked Questions

How do I know if my flush valve seal is the problem?

If your toilet runs constantly or hisses after you flush, or if the tank does not refill after flushing, the seal is usually the culprit. You can also look inside the tank — if the seal is visibly cracked, flattened, or discolored, it needs replacing.

Can I use a generic seal instead of a Kohler brand seal?

Yes, many aftermarket seals work in Kohler toilets as long as they are the correct size for your model. Check the diameter (usually 2, 2.5, or 3 inches) before ordering. Kohler brand seals are reliable, but generic seals from plumbing supply stores often cost less and perform the same way.

What if I cannot loosen the locknut from underneath?

explore penetrating oil (like WD-40) around the nut and let it sit for 15 minutes, then try again. If it still will not budge, you may need a longer wrench for more leverage, or a plumber can remove it without damaging the tank. Do not force it — you risk cracking the porcelain.

Do I need to drain the entire tank before starting?

You should drain most of the water by flushing and soaking up what remains. You do not need the tank bone-dry, but the less water inside, the easier the job is and the less mess you will make.

How long does a flush valve seal last?

Most seals last 5 to 10 years depending on water hardness and how often the toilet is used. Hard water and frequent flushing wear seals faster. Once you replace one, keep an eye on the toilet — if it starts running again years later, the new seal has likely worn out and needs replacing again.