Replacing a Fluidmaster fill valve takes 15 to 30 minutes and costs $15 to $40 for the part

A Fluidmaster fill valve is the mechanism that refills your toilet tank after you flush. When it fails, your toilet either runs constantly, fills slowly, or won't fill at all. Replacing it yourself means turning off the water, draining the tank, unbolting the old valve, and screwing in the new one — no special tools required beyond an adjustable wrench and a bucket.

The job is straightforward because Fluidmaster valves are designed for homeowner replacement. You do not need a plumber unless your shutoff valve is stuck or the tank itself is cracked. Most people complete this in one sitting without calling anyone.

Key Takeaways

  • Turn off the water at the shutoff valve behind or below the toilet, then flush to empty the tank completely before you start.
  • The fill valve bolts to the underside of the tank from inside; you will need an adjustable wrench to loosen the nut underneath and remove it.
  • Fluidmaster makes several models (400, 401, 402) that fit most standard toilets, but measure your tank opening or bring the old valve to the store to confirm the size.
  • The new valve comes with a rubber gasket and locknut; hand-tighten first, then use the wrench to snug it down without over-tightening, which can crack the tank.
  • After installation, turn the water back on, let the tank fill, and adjust the float arm or clip so the water stops at the fill line marked inside the tank.

What you will need before you start

Gather these items in one place: an adjustable wrench, a bucket or towel to catch water, a flashlight if your bathroom is dim, and the replacement Fluidmaster valve. Most hardware stores stock the 400 and 401 models; the 400 is the most common for standard tanks.

Before you buy, check your current valve or measure the opening on the underside of your tank. Fluidmaster valves come in two sizes: standard (for most toilets) and compact (for tight spaces). If you are unsure, take a photo of the tank bottom or bring the old valve with you to the store.

You do not need a plumber's license, a special wrench, or any sealant. The rubber gasket on the new valve creates the seal; Teflon tape or plumber's putty is not necessary and can actually cause leaks.

Turn off the water and drain the tank

Locate the shutoff valve on the water line behind or below the toilet. Turn it clockwise until it stops. If the valve is stiff or will not turn, do not force it — call a plumber, because a stuck valve can break and flood your bathroom.

Once the water is off, flush the toilet to empty the tank. Use a towel or sponge to soak up any remaining water in the bottom. This step prevents water from spilling onto your floor when you remove the old valve.

If water continues to drip from the shutoff valve after you turn it off, the valve itself is leaking. You can still proceed, but place a bucket under the tank to catch drips while you work.

Remove the old fill valve

Look at the underside of the tank. You will see the fill valve sticking down through the bottom with a large nut holding it in place. This nut is called the locknut. Use your adjustable wrench to loosen it by turning counterclockwise. You may need to hold the valve body steady with your other hand so it does not spin.

Once the locknut is loose enough to turn by hand, unscrew it the rest of the way and set it aside. Lift the old valve out of the tank from the inside. Some water may still be trapped in the valve body; let it drain into your bucket.

Before you throw the old valve away, look at the rubber gasket. If it is cracked, hardened, or visibly damaged, that is why your toilet was leaking or running. The new valve includes a fresh gasket, so you will not reuse the old one.

Install the new Fluidmaster valve

Take the new valve out of the box and check that it includes the rubber gasket and locknut. Slide the rubber gasket onto the valve stem (the part that goes through the tank bottom). The gasket should sit flat against the base of the valve.

Lower the valve through the opening in the tank bottom. Make sure the gasket is positioned correctly and the valve sits flush against the tank. From underneath, thread the locknut onto the valve stem by hand first. This prevents cross-threading, which can strip the threads and cause leaks.

Once the locknut is hand-tight, use your adjustable wrench to tighten it another quarter turn. Do not over-tighten. A cracked tank is expensive to replace, and the gasket seals the connection — extra force does not make it tighter. If water leaks after you turn the water back on, tighten the nut another eighth of a turn, not a full turn.

Reconnect the water supply and adjust the float

Turn the shutoff valve counterclockwise to restore water flow. Watch the tank fill. It should take 30 seconds to two minutes to reach the fill line. If it fills much slower, the valve may not be seated correctly; turn the water off and check that the gasket is flat and the locknut is snug.

Once the tank is full, locate the float arm or float clip on the valve. This is the part that rises with the water and shuts off the valve when the tank is full. The fill line is usually marked inside the tank with a line or label. Adjust the float so the water stops just below that line — typically about one inch below the overflow tube.

On Fluidmaster 400 and 401 models, you adjust the float by bending the arm slightly or sliding the clip up and down the valve stem. Consult the instruction sheet that came with your valve for the exact method. After you adjust it, flush the toilet and watch the tank refill to confirm the water stops at the right level.

Check for leaks and test the flush

Let the toilet sit for five minutes, then check underneath the tank for drips. If you see water pooling around the locknut, turn off the water and tighten the nut another eighth of a turn. Wait five minutes and check again. Repeat until the leak stops.

Flush the toilet three times and listen for the tank to refill normally each time. The valve should shut off with a quiet click or hiss, not a grinding sound. If you hear grinding or the valve does not shut off after two minutes, turn off the water and check that the float is adjusted correctly and moving freely.

If the tank still runs or fills slowly after adjustment, the new valve may be defective. Most hardware stores accept returns within 30 days, so keep your receipt. Bring the valve back and exchange it for another unit.

Frequently Asked Questions

Do I need to turn off the water to replace the fill valve?

Yes. If you do not turn off the water at the shutoff valve, water will spray out of the tank opening when you remove the old valve. The shutoff valve is always located on the water line behind or below the toilet.

What if the locknut will not loosen?

explore penetrating oil (like WD-40) around the nut and wait 15 minutes. Try again with the wrench. If it still will not budge, do not force it — you risk cracking the tank. Call a plumber at this point.

Can I use the old gasket with a new valve?

No. Always use the new gasket that comes with the replacement valve. Old gaskets harden over time and lose their seal, which is usually why the toilet was leaking in the first place.

How tight should the locknut be?

Hand-tight plus a quarter turn with the wrench. If water leaks after you turn the water back on, tighten it an eighth of a turn more. Over-tightening can crack the porcelain tank and is the most common mistake homeowners make.

What if water keeps running after I adjust the float?

Make sure the float arm or clip moves freely and is not stuck against the valve body or overflow tube. If it moves freely but the water still runs, the new valve may have a defect. Return it to the store for an exchange.