What you need to know before you start

A Kohler flush valve seal is a rubber or silicone ring that sits inside your toilet tank and keeps water from leaking into the bowl when the tank is full. When it wears out, water runs constantly into the bowl, your water bill climbs, and you hear the tank refilling on its own. Replacing the seal takes about 30 minutes and costs $5 to $15 for the part itself — no plumber needed.

The seal lives inside the flush valve, which is the large mechanism at the bottom center of your tank. You will need to turn off the water, drain the tank, and unscrew the flush valve from underneath. The seal itself straightforward pulls out and a new one pushes in. The hardest part is usually reaching the bolts under the tank, not the seal replacement itself.

Key Takeaways

  • Turn off the water at the shutoff valve behind or below the toilet, then flush to empty the tank completely.
  • The flush valve bolts are underneath the tank; you will need a wrench or adjustable pliers and access from below, so a bucket or towel under the tank catches remaining water.
  • Once the flush valve is removed, the old seal pulls straight out — it is not glued or fastened — and the new one pushes in by hand.
  • Kohler flush valve seals come in standard sizes, but bring your old seal to the hardware store or measure the diameter to buy the exact match.
  • After reassembly, the tank may take a few fill cycles to seal properly; if water still runs, the seal may not be seated evenly.

Gather your tools and materials

You will need an adjustable wrench or a set of open-end wrenches (usually 7/8 inch and 1 1/16 inch for Kohler valves), a flathead screwdriver, a bucket or towel, and a replacement flush valve seal. Kohler seals are sold at hardware stores, home centers, and online under model numbers like 1188170 or 1188171 — the number depends on your toilet model. If you do not know your model, bring the old seal with you or look on the underside of the tank for a model sticker.

Have a bucket or old towel ready under the tank before you start. Even after you flush, a small amount of water stays in the valve and will spill when you unscrew the bolts. Wear old clothes or an apron because you will be working underneath the tank and water will drip on you.

Turn off water and drain the tank

Locate the shutoff valve on the water line behind or below the toilet. Turn it clockwise until it stops — you should feel resistance but do not force it. If the valve is stuck or will not turn, you can still proceed, but water will run out when you disconnect the supply line. Flush the toilet and hold the handle down for three seconds to empty the tank as much as possible.

If water is still pooling in the tank, use a sponge or small cup to soak it up and wring it into the bowl. You do not need the tank bone-dry, but the less water inside, the less mess when you remove the valve. Once the tank is mostly empty, you are ready to work underneath.

Remove the flush valve from underneath the tank

Lie on your back or crouch under the toilet and look at the bottom of the tank. You will see a large nut (usually brass or plastic) in the center, with two smaller bolts on either side. The center nut holds the flush valve in place; the side bolts hold the tank to the bowl. You only need to remove the center nut and the bolts that hold the valve itself.

Place your bucket under the valve first. Using your wrench, turn the center nut counterclockwise while holding the valve steady from inside the tank with your other hand or a screwdriver. Once the nut is loose, unscrew it by hand. You may see a rubber or foam washer under the nut — keep this; you will reinstall it. Gently pull the flush valve down and out of the tank. Water will drip, so let it drain into your bucket.

Remove and replace the seal

Once the valve is out, look at the bottom. You will see a rubber or silicone ring — that is your seal. It may be sitting in a groove or straightforward resting against the valve body. Pull it straight out with your fingers. It should come free easily; if it is stuck, run your fingernail around the edge to loosen it, but do not pry hard enough to damage the valve itself.

Inspect the valve seat (the surface where the seal sits) for cracks, mineral buildup, or debris. If you see white crusty deposits, wipe them away with a damp cloth. If the valve seat is cracked or pitted, the new seal will not hold water and you will need to replace the entire valve — but this is rare. Once the seat is clean, push the new seal into place by hand. It should sit flush and even all the way around. Rotate it gently to make sure it is seated evenly.

Reinstall the flush valve and refill the tank

Slide the flush valve back up into the tank opening from underneath. Make sure the seal is still in place and has not shifted. From underneath, thread the center nut back on by hand first, then tighten it with your wrench. Tighten firmly but do not over-tighten — you want it snug, not stripped. If you removed the rubber washer, make sure it is under the nut before you tighten.

Turn the shutoff valve counterclockwise to restore water to the tank. The tank will fill; listen for the fill valve to shut off when the tank reaches the waterline. Once full, flush the toilet and watch the tank refill. The seal may take one or two fill cycles to seat completely — this is normal. If water still runs continuously into the bowl after the second flush, the seal may not be seated evenly. Turn off the water again, remove the valve, and check that the seal is sitting flat all the way around.

Common mistakes to avoid

The most common error is over-tightening the center nut, which can crack the plastic tank or strip the threads. Tighten until snug, then stop. Another mistake is installing the seal upside down or twisted — it should sit flat and even in its groove. If you hear water running after reassembly, do not assume the seal is bad; check first that it is seated properly by removing the valve again and rotating the seal to make sure it is centered.

Do not forget the rubber washer under the center nut. Without it, the nut will not seal properly and water will leak from underneath the tank. If you lose the old washer, a new one costs a dollar or two at any hardware store. Finally, if the old seal is stuck or crumbling, soak it in warm water for a few minutes to soften it before pulling it out.

Frequently Asked Questions

How do I know if my flush valve seal is the problem and not something else?

If water runs constantly into the bowl without you flushing, and the fill valve (the smaller mechanism on the side of the tank) is shutting off normally, the flush valve seal is almost certainly worn. You can confirm by adding food coloring to the tank water — if the color seeps into the bowl without flushing, the seal is leaking. A bad fill valve will cause the tank to refill on its own, which is a different problem.

Can I use any rubber seal or does it have to be a Kohler brand?

The seal must match your valve size and shape, but it does not have to be Kohler brand. Generic replacement seals sold at hardware stores work fine as long as the diameter and thickness match your old seal. Bring the old seal with you to the store and compare, or buy a Kohler seal if you are unsure — they are inexpensive and may provide to fit.

What if the flush valve itself is cracked or damaged?

If the valve body is cracked, a new seal will not stop the leak. You will need to replace the entire flush valve, which costs $20 to $40 and takes about an hour. The process is similar — remove the old valve and install a new one — but you will also need to install a new seal on the new valve. If you are unsure whether the valve is damaged, remove it and inspect it under good light before buying a replacement.

Do I need to turn off the water if I am just replacing the seal?

Yes. Even though you are only removing the seal, you have to remove the entire flush valve to reach it, and that requires disconnecting it from the tank. Turning off the water first prevents water from spraying out when you loosen the bolts and keeps the tank from refilling while you work.

How long does a Kohler flush valve seal usually last?

Most seals last five to seven years depending on water hardness and use. Hard water with mineral deposits can wear seals faster. If you replace the seal and it fails again within a year, the valve seat may be damaged or mineral buildup may be preventing a good seal — in that case, cleaning the valve seat or replacing the entire valve may be necessary.