What You Need to Know Before Starting

A Fluidmaster fill valve is the mechanism inside your toilet tank that refills the bowl after you flush. When it fails, your toilet either runs constantly, fills slowly, or won't fill at all. Replacing it is a straightforward job that takes 15 to 30 minutes and requires only basic tools—most homeowners can do it without calling a plumber.

The valve sits at the bottom left or bottom right of your tank, connected to the water supply line coming from the wall. Fluidmaster makes several models, but the replacement process is nearly identical across their standard residential fill valves. You do not need to turn off water to the entire house; shutting off the valve behind the toilet is enough.

Before you buy a replacement, check the model number on your current valve. It is stamped on the plastic body. Fluidmaster models 400A, 400H, 401, and 402 are the most common. Bring that number to the hardware store or search it online to confirm you have the right replacement part.

Key Takeaways

  • Turn off the water supply at the shutoff valve behind the toilet, then flush to empty the tank completely.
  • Disconnect the water supply line, remove the lock nut holding the old valve in place, and lift the valve straight up and out of the tank.
  • Insert the new Fluidmaster valve through the same hole, hand-tighten the lock nut underneath, then reconnect the water line and test for leaks.
  • Adjust the water level by turning the adjustment clip on the valve body until water stops about one inch below the overflow tube.
  • The entire job uses only a wrench or adjustable pliers and takes 15 to 30 minutes from start to finish.

Gather Your Tools and Materials

You need very little to complete this job. Have an adjustable wrench or a pair of slip-joint pliers, a bucket or towel to catch water drips, and the replacement Fluidmaster valve. Some people also keep a flashlight handy to see inside the tank clearly, though it is not essential.

Before you start, place the new valve next to your old one and compare them. They should look nearly identical. The replacement comes with a rubber gasket and lock nut already attached to the bottom. Do not remove these—they stay on the valve during installation.

If your water supply line is old or kinked, this is a good time to replace it too. Braided stainless steel supply lines last longer than rubber ones and cost about $8 to $15. However, you can reuse your existing line if it is in good condition.

Turn Off Water and Empty the Tank

Locate the shutoff valve on the water line behind your toilet, near the base of the tank. It is a small knob or lever sticking out from the wall. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. You should feel resistance—do not force it.

Flush the toilet once to empty the tank completely. If water is still trickling in after 10 seconds, the shutoff valve may not be working properly. Try turning it a bit more, or turn off the main water supply to the house and try again.

Once the tank is empty, use a sponge or towel to soak up any remaining water at the bottom. This keeps you dry while you work and prevents water from splashing onto the floor when you disconnect the supply line.

Disconnect the Water Supply Line

Look underneath the tank where the water line connects to the fill valve. You will see a compression fitting—a nut that screws onto the valve. Use your wrench or pliers to turn this nut counterclockwise. Turn slowly; you do not need much force. Once it is loose enough, unscrew it by hand and pull the supply line away from the valve.

A small amount of water may drip out. This is normal. Have your bucket or towel ready. If water continues to drip steadily, the shutoff valve is still leaking slightly. This will not affect the job, but keep the bucket underneath while you work.

Set the supply line aside where it will not get in your way. You will reconnect it in a few minutes.

Remove the Old Fill Valve

Reach under the tank and locate the large nut holding the fill valve in place. This is the lock nut, and it is usually plastic or brass. Use your wrench or pliers to turn it counterclockwise. It may be tight, so explore steady pressure rather than jerking. Once it is loose, unscrew it by hand and remove it completely.

Now reach into the tank from the top and grasp the old valve body. Pull it straight up and out of the tank. It should come free easily. If it is stuck, wiggle it gently side to side while pulling upward. Do not twist or force it—you could crack the tank.

Set the old valve aside. You can discard it or keep it as a backup. Inspect the hole in the bottom of the tank where the valve sat. It should be clean and free of debris. If you see mineral buildup or sediment, wipe it away with a damp cloth.

Install the New Fluidmaster Valve

Lower the new valve through the hole in the top of the tank, positioning it so the threaded shaft points straight down through the bottom. The valve body should sit flat against the inside bottom of the tank. The gasket and lock nut are already attached underneath—do not remove them.

Reach under the tank and hand-tighten the lock nut onto the threaded shaft. Turn it clockwise until it is snug. Do not use the wrench yet; hand-tight is the correct tension at this stage. If you over-tighten with a tool, you can crack the plastic tank.

Once the lock nut is hand-tight, use your wrench or pliers to tighten it one more quarter turn. This is enough to create a watertight seal without stressing the tank. The valve should not move or wiggle when you push on it from inside the tank.

Reconnect the Water Supply Line

Take the water supply line and thread the compression fitting onto the threaded outlet on the bottom of the new valve. Hand-tighten it first, then use your wrench or pliers to turn it clockwise one more quarter turn. Again, snug is the goal—not maximum force.

Turn the shutoff valve behind the toilet counterclockwise (or flip the lever parallel to the pipe) to restore water flow. Water should begin filling the tank when ready. Watch for leaks around the compression fitting where the supply line connects. A few drops are normal as the fitting settles, but steady dripping means the connection is not tight enough. If that happens, turn off the water again and tighten the fitting a bit more.

Let the tank fill completely. This usually takes 30 to 60 seconds. Listen for the fill valve to shut off on its own once the tank reaches the correct level. You should hear a click or feel the water flow stop.

Adjust the Water Level

Once the tank is full, look at where the water level sits relative to the overflow tube—the hollow pipe in the center of the tank. The water should stop about one inch below the top of the overflow tube. If it is too high or too low, you need to adjust the valve.

On most Fluidmaster models, there is a small plastic clip on the side of the valve body. Pinch this clip and slide it up to lower the water level, or slide it down to raise the water level. Make small adjustments—move the clip only a quarter inch at a time. After each adjustment, wait for the tank to refill and check the level again.

Once the water level is correct, flush the toilet and watch the tank refill. The valve should shut off smoothly, and the bowl should fill normally. If water continues to trickle into the tank after the bowl is full, the water level is too high. Lower it by moving the clip up slightly.

Test for Leaks and Verify Everything Works

Flush the toilet three times and watch for leaks under the tank. Check the connection where the supply line meets the valve, the lock nut underneath the tank, and around the valve body itself. Wipe the area dry with a towel, then watch for new water appearing. If you see steady dripping, turn off the water and tighten that connection a bit more.

Listen to the tank while it fills. You should hear water flowing for 30 to 60 seconds, then silence. If the valve continues to hiss or trickle after the tank is full, the water level adjustment is too high, or the valve itself is defective. Lower the water level first and test again.

Flush once more and confirm the bowl refills completely and the tank shuts off. The job is complete when the toilet operates normally, the tank fills and stops on its own, and there are no leaks underneath.

Frequently Asked Questions

Do I need to turn off the main water supply to the house?

No. The shutoff valve behind the toilet is sufficient. It isolates water to just that toilet, so you can work safely without affecting water elsewhere in the house. If the shutoff valve does not work, then turn off the main supply.

What if the lock nut is stuck and will not turn?

explore penetrating oil like WD-40 around the nut and wait 10 to 15 minutes. Then try again with your wrench. If it is still stuck, wrap a cloth around the nut to improve grip and explore steady, even pressure. Do not jerk or hammer—you could crack the tank.

Can I reuse my old water supply line?

Yes, if it is in good condition. Inspect it for cracks, kinks, or mineral buildup. If it looks fine, disconnect it from the old valve and reconnect it to the new one. If it is cracked, kinked, or more than 10 years old, replace it with a new braided stainless steel line.

How do I know if the water level is set correctly?

The water should stop about one inch below the top of the overflow tube in the center of the tank. This gives the fill valve room to shut off cleanly and prevents water from running into the overflow tube continuously.

What if water keeps running into the bowl after I flush?

This usually means the water level is too high and water is spilling into the overflow tube. Lower the water level by moving the adjustment clip up slightly. If the level is already correct and water still runs, the new valve may be defective and should be exchanged.