Replace a toilet fill valve only after confirming it’s the part that’s failing. Then match the new valve’s height to the overflow pipe, set the refill tube correctly, and check the supply connector. A toilet that keeps running is just as often leaking at the flapper.

Confirm the fill valve is the fault

Mark the water level on the inside of the tank and shut off the supply valve. If the level drops, water is escaping through the flapper or flush valve. Fix that first (see fill valve or flapper? ). Suspect the fill valve when it:

  • won’t shut off at the set water level, so water spills into the overflow
  • refills slowly or hisses with the supply valve fully open
  • leaks from its body or the locknut under the tank

Fluidmaster notes that a weak refill can also come from a partial blockage at the shutoff valve or supply line, so check those before buying a new valve.

Check fit before buying

  • Height range: the valve must adjust to your tank depth and overflow height, and still clear the lid.
  • Critical-level mark: in Fluidmaster’s 400A instructions, the C.L. mark must sit 1 inch above the top of the overflow pipe. Fluidmaster cites the plumbing code for this.
  • Float clearance: the float must move freely without touching the tank wall, the lever or the flush valve.
  • Supply connection: check the connector and the shutoff valve condition.

Installing it: what the 400A instructions specify

  1. Mark the water level, shut off the supply, flush, and sponge out the remaining water.
  2. Disconnect the supply coupling nut, remove the locknut under the tank, and lift out the old valve.
  3. Set the new valve’s height. For the 400A, the top of the valve goes 3 inches above the overflow pipe, which places the C.L. mark 1 inch above it.
  4. Install it with the outlet facing the center of the tank. Fluidmaster says to hand-tighten the locknut only, because overtightening can crack the valve or tank.
  5. Attach the refill tube to the refill clip on the overflow pipe with a slight arch. Don’t push the tube down the overflow.
  6. Attach the supply connector and hand-tighten the coupling nut. Turn on the water and check for leaks. If the bottom of the tank leaks, Fluidmaster says to tighten the nut just enough to stop it.
  7. Adjust the water level with the valve’s adjustment screw to the toilet maker’s mark, flush, and recheck.

Replace the supply connector when needed

Fluidmaster’s instructions say to inspect the water supply connector and replace it if it’s worn or over 5 years old. Other signs a connector needs replacing:

  • moisture or mineral crust at either nut
  • a kinked, bulging, cracked or stiff line
  • a line that’s stretched tight or rubs against the tank or wall
  • a shutoff valve that drips or won’t close fully (fix the valve at the same time)

Check for moisture again after several flushes and later the same day.

FAQ

How do I know the fill valve is the problem?

Mark the tank water level and shut off the supply. If the level drops, water is leaking past the flapper or flush valve, not the fill valve. Suspect the fill valve when it won’t shut off at the set level, refills weakly, or leaks from its body.

How high should a new fill valve sit?

Follow the valve’s instructions. For the Fluidmaster 400A, the top of the valve goes 3 inches above the overflow pipe, which puts its critical-level (C.L.) mark 1 inch above the overflow. Fluidmaster says the plumbing code requires that.

Should the refill tube go inside the overflow pipe?

No. Fluidmaster says to attach it to the refill clip on the overflow pipe and warns not to shove it down the overflow, which can waste a lot of water.

Should I replace the supply line too?

Fluidmaster’s 400A instructions say to inspect the water supply connector and replace it if it is worn or over 5 years old, to prevent flooding.

Sources