Symptoms that point to the fill valve
Four symptoms diagnose a failing fill valve: (1) water keeps refilling forever after a flush — never stops; (2) tank fills very slowly, taking 5+ minutes; (3) water level rises above the top of the overflow tube and you hear constant draining; (4) the fill valve makes a high-pitched whine or hammers when filling.
Tools and parts
- Universal fill valve (Fluidmaster 400A — $9; Korky 528 — $12)
- Adjustable wrench or 1-inch nut wrench
- Sponge and bucket
- Old towel
The full procedure
1. Shut off water and drain tank. Close angle stop, flush, sponge remaining water.
2. Disconnect the supply line. Hand-loosen the supply nut where it enters the bottom of the tank. Catch the drip with the towel.
3. Remove the lock nut. Under the tank, the fill valve is secured by a large plastic or brass lock nut. Loosen with the wrench (or by hand if the previous installer didn\'t over-tighten). Lift the old fill valve straight up out of the tank.
4. Adjust the new valve height. Fluidmaster 400A and similar universal valves telescope — you adjust the length so the top of the valve sits at least 1 inch above the overflow tube. Set this before installation. The valve marking "CL" (Critical Level) is the line that must sit above the overflow.
5. Install the new valve. Drop it into the same hole the old one came out of. From below the tank, screw on the lock nut by hand — then tighten 1/4 turn with the wrench. Do not over-tighten; you\'ll crack the tank.
6. Connect the refill tube. The flexible black tube goes from the new fill valve to the top of the overflow tube. Insert it into the supplied clip on top of the overflow tube — the tube outlet should be above the overflow tube top, not below. (Below creates a siphon that can pull tank water back into the supply.)
7. Connect supply line and turn on water. Reattach the supply nut hand-tight, then 1/4 turn with the wrench. Open the angle stop slowly.
8. Set the water level. Most valves have a float adjustment screw or slider. Adjust until the tank water sits exactly at the "WL" (Water Line) marking on the inside of the tank — typically 1 inch below the top of the overflow tube. Flush 2-3 times to confirm.
Common gotchas
Refill tube too long. If the tube extends down into the overflow tube past the rim, the tank can siphon empty silently. Cut it to length so the outlet sits 1/2 inch above the overflow rim.
Water hammer. If the fill valve clunks loudly when closing, install a mini water-hammer arrestor on the supply line ($10 in-line fitting).
Slow fill that won\'t resolve. Almost always supply-line restriction, not the new valve. Remove the supply line and flush debris from the angle stop.