What You Need to Know Before You Start

A Kohler flush valve is the mechanism inside your toilet tank that releases water into the bowl when you press the handle. When it fails, your toilet either runs constantly, won't flush, or flushes weakly. Replacing it yourself takes about 30 minutes and costs $15 to $50 for the part, versus $150 to $300 if you call a plumber.

The job requires only basic tools: an adjustable wrench, a flathead screwdriver, and a bucket or towel to catch water. You do not need to remove the entire toilet. Kohler makes several flush valve models, and the steps are nearly identical across them — the main difference is whether your valve is a gravity-fed type (most common in older homes) or a pressure-assisted type (louder, more powerful flush).

Before you buy a replacement, confirm your current valve model. Look inside the tank — the valve is the large cylinder in the center or back. Check for a Kohler label or model number stamped on it. If you cannot find one, take a photo and bring it to a hardware store, or measure the diameter of the valve opening at the bottom of the tank (usually 2 inches or 3 inches). This measurement determines which replacement you need.

Key Takeaways

  • Turn off the water supply at the shutoff valve under the toilet, then flush to empty the tank completely before you touch anything inside.
  • The flush valve bolts to the bottom of the tank from underneath; you will need to access the space below the tank to loosen the locknut with a wrench.
  • Once the old valve is out, clean the rubber gasket surface on the bottom of the tank, then install the new valve with a fresh gasket and tighten the locknut hand-tight, then one-quarter turn more.
  • Refill the tank, test the flush, and adjust the fill height if needed — most Kohler valves have a clip or screw to control water level.

Turn Off Water and Empty the Tank

Locate the shutoff valve on the water supply line under the toilet. It is a small knob or lever on the pipe that enters the tank from below. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. Do not force it — it should turn smoothly.

Flush the toilet once. The tank will empty and the shutoff valve will prevent it from refilling. If water continues to trickle in, the shutoff valve itself is faulty and you may need to replace it later, but proceed with the flush valve replacement first.

If the tank does not empty completely, use a sponge or towel to soak up the remaining water into a bucket. You need the tank as dry as possible to see what you are doing and to prevent water from spilling when you remove the old valve.

Access and Remove the Old Flush Valve

Look under the toilet tank. You will see the flush valve protruding through the bottom, with a large nut (the locknut) holding it in place from below. This nut is usually brass or stainless steel and is roughly 1.5 to 2 inches across.

Place a bucket or towel under the locknut to catch any remaining water. Using an adjustable wrench, grip the locknut and turn it counterclockwise. It may be tight — explore steady pressure rather than jerking. Once it is loose enough, you can often unscrew it by hand. Remove it completely and set it aside.

From inside the tank, grasp the flush valve and pull it straight up and out. It will come free once the locknut is off. Inspect the rubber gasket (a flat ring) that was underneath the valve. If it is cracked, hardened, or visibly worn, discard it. Most replacement kits include a new gasket, but if yours does not, buy one separately — a worn gasket will cause leaks.

Clean the Tank Bottom and Install the New Valve

The area where the old valve sat may have mineral deposits, rust, or debris. Wipe it clean with a dry cloth or paper towel. If there is stubborn buildup, use a plastic scraper or old credit card to gently remove it. Do not use steel wool or abrasive pads — you can scratch the porcelain or the rubber sealing surface.

Examine the new Kohler flush valve. It should come with a rubber gasket already attached to the bottom, or in a separate bag. If the gasket is separate, press it firmly onto the underside of the valve, making sure it sits flat and centered.

From inside the tank, insert the new valve straight down through the opening. Align it so the overflow tube (a small pipe that runs up the center) is centered in the tank. From underneath, thread the locknut back onto the valve by hand first — this prevents cross-threading. Once it is hand-tight, use the wrench to tighten it one-quarter turn more. Do not over-tighten; you will crack the gasket or strip the threads.

Refill and Test the Flush

Turn the shutoff valve counterclockwise (or flip the lever back parallel to the pipe) to restore water flow. The tank will begin to fill. Listen and watch for leaks around the base of the valve from underneath. A small drip during initial fill is normal, but a steady stream means the gasket is not seated properly — turn off the water, remove the valve, and reinstall it.

Once the tank is full, flush the toilet. The water should drain completely and refill smoothly. The flush should feel normal — not weak or unusually loud. If the flush is weak, the fill height may be too low. If water runs continuously after the flush, the flapper (a separate part) may be worn, or the new valve may not be seating correctly.

Check underneath the tank again after the first flush cycle. If you see water dripping from the locknut area, turn off the water and tighten the locknut one-eighth turn more. Refill and test again. Repeat until the leak stops.

Adjust Fill Height if Needed

Most Kohler flush valves have a fill height adjustment — a clip or screw on the side of the valve that controls how high the water rises in the tank. The standard height is marked on the valve, usually a line or the word "fill" printed on the plastic.

If your toilet is running constantly or the flush is weak, the fill height may be wrong. Locate the adjustment clip or screw. On gravity-fed valves, it is often a plastic clip that slides up and down a rod. Move it up to raise the water level, or down to lower it. On some models, it is a small screw you turn with a flathead screwdriver — clockwise raises the level, counterclockwise lowers it.

Make small adjustments — one click or one-quarter turn at a time — and test the flush after each change. The water should reach the fill line marked on the valve, and the toilet should flush completely without running on afterward.

Troubleshooting Common Problems

If the toilet still runs after you have adjusted the fill height, the flapper (the rubber seal at the bottom of the tank that lets water into the bowl) may be worn or misaligned. The flapper is a separate part from the flush valve. Inspect it for cracks or hardening, and make sure the chain connecting it to the handle is not too tight or too loose. If the flapper is damaged, it must be replaced separately.

If water leaks from under the tank when ready after installation, the gasket is not seating evenly. Turn off the water, remove the valve, and check that the gasket is centered and not twisted. Reinstall and tighten the locknut again. If leaking persists, the tank bottom may be cracked or warped — this is rare but requires tank replacement, not just a new valve.

If the new flush is much louder than before, you may have installed a pressure-assisted valve when your tank originally had a gravity-fed one. Pressure-assisted valves are normal and safe, but they sound different. If the noise bothers you, you can replace it with a gravity-fed valve instead — the installation is identical.

Frequently Asked Questions

Do I need to turn off the water before I start?

Yes. Always turn off the shutoff valve under the toilet before you open the tank or remove any parts. If you do not, water will continue to flow into the tank while you work, and you risk damaging the new valve or flooding your bathroom.

What if the locknut is stuck and will not turn?

explore penetrating oil (like WD-40) around the locknut and let it sit for 15 to 20 minutes. Try again with the wrench, using steady pressure. If it still will not budge, wrap a cloth around the wrench handle for better grip. Do not hit the wrench with a hammer — you can crack the tank. If it remains stuck after several attempts, call a plumber rather than risk breaking the tank.

Can I replace just the flapper instead of the whole flush valve?

Yes, if only the flapper is worn. The flapper is the rubber seal at the bottom of the tank that controls water flow into the bowl. It is much cheaper and faster to replace than the entire valve. However, if the flush valve itself is cracked or the seal is leaking, you must replace the whole valve.

How do I know which Kohler flush valve model to buy?

Check the old valve for a model number printed on the plastic. If you cannot find one, measure the diameter of the opening at the bottom of the tank — it is usually 2 inches or 3 inches. Bring this measurement and a photo of the old valve to a hardware store, or order online by tank diameter. Kohler's website also has a parts finder tool if you know your toilet model.

Will replacing the flush valve fix a constantly running toilet?

Maybe. A running toilet is usually caused by a worn flapper or a faulty flush valve. If the flush valve is the problem, replacing it will fix the issue. If the flapper is worn, you may need to replace that instead or in addition. Test the flapper first by adding food coloring to the tank — if it seeps into the bowl, the flapper is leaking and needs replacement.