What you need to do before you start

Replacing a Fluidmaster fill valve takes about 30 minutes and requires no special tools beyond what most households have. The valve sits inside the tank and controls the water level; when it fails, the toilet either runs constantly or won't fill. You can do this yourself by turning off the water, draining the tank, unbolting the old valve, and installing the new one in its place.

Before you begin, turn off the water supply at the shutoff valve behind or below the toilet. Flush once to empty the tank, then use a sponge or towel to soak up any remaining water at the bottom. This keeps you dry and prevents water from spilling onto your floor.

Buy a replacement Fluidmaster fill valve that matches your tank opening size — most are either 1.25 inches or 1.5 inches. Fluidmaster makes several models; the 400A is the most common and works in nearly all tanks. Bring a photo of your current valve or the tank opening to the hardware store if you're unsure which size you need.

Key Takeaways

  • Turn off the water at the shutoff valve behind the toilet and flush to empty the tank completely before removing anything.
  • The fill valve bolts to the bottom of the tank from the inside; you'll need to reach under the tank from below to unscrew the locknut.
  • The new valve slides in from the top of the tank and is secured with the same locknut you removed from the old one.
  • After installation, adjust the water level by turning the adjustment clip or screw on the new valve until water stops about one inch below the overflow tube.
  • Test the valve by turning the water back on and letting the tank fill, then flush to confirm it refills and shuts off correctly.

Removing the old fill valve from underneath the tank

Reach under the toilet tank from below and locate the locknut — a large nut that holds the fill valve in place. It's on the underside of the tank, directly below where the valve sits inside. Use an adjustable wrench or pliers to turn this nut counterclockwise. You may need to hold the valve steady from inside the tank with your other hand so it doesn't spin.

Once the locknut is loose enough to turn by hand, unscrew it completely and set it aside. The old fill valve will now lift out from the top of the tank. If it's stuck, gently rock it side to side as you pull. Discard the old valve and the rubber washer that was underneath it.

Before you install the new valve, wipe the area around the opening in the bottom of the tank with a clean cloth. Remove any mineral deposits or debris. This ensures a watertight seal when you install the new valve.

Installing the new Fluidmaster valve

Slide the new fill valve down through the opening in the top of the tank. The threaded portion should poke through the bottom. From underneath, slide the rubber washer that came with the new valve onto the threaded portion, then hand-screw the locknut back on. Tighten it firmly with your wrench, but do not over-tighten — you only need it snug enough that water won't leak around the base.

The valve should now be find and centered in the tank opening. Check that it's not tilted or crooked by looking at it from above. If the overflow tube (the hollow pipe in the center of the tank) is in the way, you may need to adjust the valve's position slightly, but most Fluidmaster valves are designed to fit around it.

Connecting the water supply line and adjusting the water level

Reconnect the water supply line to the fill valve by hand-tightening the connection nut. The line screws onto the bottom of the valve. Once hand-tight, use an adjustable wrench to turn it another quarter turn — just enough to stop any leaks. Do not over-tighten, as this can crack the fitting.

Turn the water supply back on slowly at the shutoff valve. Watch the tank fill and listen for the fill valve to shut off when the water reaches the correct level. Most Fluidmaster valves have an adjustment clip or screw on the side. If the water level is too high or too low, turn this adjustment until water stops about one inch below the top of the overflow tube.

Flush the toilet and watch the tank refill. The valve should shut off cleanly without hissing or running on. If water continues to run into the overflow tube after the tank is full, the water level is set too high — lower it by adjusting the clip or screw downward.

Testing for leaks and confirming proper operation

Let the toilet sit for 10 minutes without flushing. Check underneath the tank and around the base of the valve for any drips or wet spots. A small amount of moisture is normal during the first fill, but steady dripping means the locknut is loose or the rubber washer is damaged. If you see dripping, turn off the water, drain the tank, and tighten the locknut another quarter turn.

Flush the toilet three times and observe the fill cycle each time. The tank should fill in 30 seconds or less, and the valve should shut off with a quiet click. If the valve hisses or gurgles after shutting off, the water level is too high — adjust it down slightly.

Check that the float arm (the part that rises and falls with the water) moves freely and doesn't catch on the overflow tube or the tank wall. If it does, loosen the locknut slightly and reposition the valve until the float has clear space to move.

Common problems after installation

If the tank fills very slowly, the water supply line may be kinked or the shutoff valve may not be fully open. Check both before assuming the new valve is faulty. If the tank won't stop filling, the water level adjustment is set too high — lower it by moving the adjustment clip down one notch at a time and testing between each change.

If water leaks from the connection where the supply line meets the valve, turn off the water and tighten that nut another quarter turn with your wrench. If it still leaks after tightening, turn off the water, drain the tank, and remove the supply line. Check that the rubber washer inside the connection nut is present and not cracked. Replace it if needed, then reconnect the line.

If water leaks from under the tank where the valve bolts through, the locknut is loose. Turn off the water, drain the tank, and tighten the locknut from underneath. If tightening doesn't stop the leak, the rubber washer may be damaged — you'll need to remove the valve again and replace the washer.

When to call a plumber instead

If the tank opening is cracked or damaged, a new fill valve won't seal properly and you'll have a persistent leak. This requires tank replacement, not just a valve swap. If you see cracks around the opening when you remove the old valve, stop and contact a plumber.

If the tank is very old and the opening threads are stripped or corroded, the locknut may not tighten enough to seal. In this case, a plumber can install a repair kit or replace the tank. If you turn the locknut and it spins freely without tightening, the threads are likely damaged.

Frequently Asked Questions

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

Yes. Always turn off the water at the shutoff valve before removing any part of the toilet. This prevents water from spraying out when you disconnect the supply line and keeps the tank empty while you work.

What size fill valve do I need?

Most residential toilets use either a 1.25-inch or 1.5-inch valve. The size refers to the opening in the bottom of the tank. Measure the opening or bring a photo to the hardware store. The Fluidmaster 400A fits most tanks and is the most common replacement.

Can I reuse the old locknut and washer?

The locknut can be reused, but always install a new rubber washer. The old washer may be cracked or compressed and won't seal properly. New washers cost less than a dollar and come with most replacement valves.

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

Water should stop about one inch below the top of the overflow tube — the hollow pipe in the center of the tank. If water reaches the overflow tube, it's set too high and will waste water. Adjust the clip or screw on the valve until the level is correct.

What if the new valve leaks from underneath the tank?

Turn off the water, drain the tank, and tighten the locknut another quarter turn from underneath. If it still leaks after tightening, the rubber washer may be damaged. Remove the valve, replace the washer, and reinstall the valve.