Niagara Toilet Fill Valve — Fluidmaster 400A Universal Drop-In

Niagara toilet fill valve replacement

The fill valve is the cylindrical mechanism inside the toilet tank that refills the tank after each flush. It connects to the supply line at the bottom of the tank and shuts off when the tank reaches the correct water level. Niagara uses the standard universal 2.5-inch shank fill valve pattern — meaning Fluidmaster 400A drops in directly at $10-$15 from any Home Depot or Lowes. Unlike Eljer (proprietary 3-inch shank), Niagara doesn't require any adapter kit or special-order OEM part.

Why fill valves fail

Fill valves are the second most-replaced toilet part (after flappers). Common failure modes:

• Diaphragm clogged with sediment: Mineral deposits, sand, or rust particles clog the internal diaphragm. Tank refills slowly or incompletely.
• Diaphragm worn: After 8-15 years, the internal rubber diaphragm degrades and the valve no longer shuts off cleanly. Continuous hissing or whistling during refill.
• Float arm failure: The plastic float arm cracks or the pivot wears out, causing the valve to shut off at the wrong water level.
• Cap seal degraded: Visible leak from the top of the fill valve (water spraying or dripping).
• Anti-siphon vent compromised: Water sprays from the valve cap during refill instead of flowing down through the refill tube.

Fluidmaster 400A — Niagara-compatible direct fit

Niagara fill valve compatibility by model

Universal 2.5-inch shank across the entire Niagara residential lineup. Fluidmaster 400A fits all of the following directly:

• Stealth Sabre 77001 (0.95 GPF vacuum-assist)
• Stealth Original
• Stealth Conserver Dual-Flush (0.95/0.5 GPF) — uses standard fill valve; the dual-flush cartridge is separate from the fill valve
• Liberty (1.28 GPF elongated)
• Liberty Round-Front
• Phantom + Phantom Round-Front
• Steaful (1.28 GPF round-front)
• Power One + Power One Plus
• Niagara One (one-piece — limited internal access; longer Fluidmaster 400A install reach)
• Niagara Compact

Replacement procedure (25 minutes)

1. Shut off water at the under-toilet shutoff valve: Turn clockwise to close.
2. Flush the toilet to empty the tank: Hold the handle down to drain fully.
3. Disconnect the supply line from the bottom of the tank: Use channel-lock pliers. The brass compression nut can be over-tight after years — apply penetrating oil if needed. Have a small bucket ready to catch dripping water.
4. Sponge out remaining tank water: Especially around the fill valve base on the inside of the tank.
5. From inside the tank, unscrew the fill valve's lock nut: This nut is on the outside-bottom of the tank, holding the fill valve in place from below. Use channel-lock pliers or an adjustable wrench. Turn counterclockwise from below.
6. Lift the old fill valve straight up out of the tank: The fill valve will lift through the shank hole. Some sediment may dislodge — leave the bucket positioned below the tank to catch.
7. Inspect the shank hole: Clean any residue from the inside surface of the tank around the shank opening. This ensures the new fill valve's gasket seats cleanly.
8. Install the Fluidmaster 400A: Insert through the shank hole from inside the tank. The Fluidmaster's threaded shank extends through to the outside-bottom of the tank.
9. Hand-tighten the lock nut from below: Get it finger-tight. Then use the wrench for a final 1/4 turn beyond finger-tight. Do NOT overtighten — overtightening cracks the porcelain at the shank hole.
10. Adjust the valve height: The Fluidmaster 400A's height is adjustable via the slider on the valve body. Set so the top of the valve sits 1 inch above the tank's water line. The water line is typically marked on the inside of the tank.
11. Connect the supply line to the bottom of the tank: Hand-tighten the brass compression nut, then 1/4 turn with the wrench. Use new washers if the old ones are deformed.
12. Position the refill tube: The small flexible tube on the side of the Fluidmaster 400A must terminate inside the overflow tube top opening. If left dangling outside the overflow tube, water wastes during refill (and doesn't refill the bowl after flush).
13. Turn the shutoff valve back on: Counterclockwise. Tank begins filling.
14. Adjust the float cup slider: The Fluidmaster 400A has a sliding float cup on the valve body. As the tank fills, the float rises and shuts off the valve. Slide the float cup up or down to set the correct water level (target: 1 inch below the top of the overflow tube).
15. Test flush 3-5 times: Verify quiet refill cycle and correct shutoff at the water line. Listen for any hissing after the valve appears to shut off — if heard, the valve isn't fully closing and the cup may need slight adjustment.

Common Niagara fill valve service mistakes

• Lock nut overtightened: Cracks tank porcelain at the shank hole. Hand-tight + 1/4 turn limit.
• Anti-siphon vent positioned below the water line: The valve can siphon dirty bowl water back into the supply line. Always position the anti-siphon vent (cap) above the overflow tube top.
• Refill tube not directed into the overflow tube: Wastes water and prevents bowl refill. The tube must terminate inside the overflow tube opening.
• Float adjustment too high: Water continuously runs into the overflow tube. Slide the float cup down to lower the shutoff level.
• Float adjustment too low: Tank doesn't fill completely; flush volume is reduced. Slide the float cup up.
• Reused old supply line: 12+ year old supply lines internally degrade. Replace at the same time as the fill valve ($5-$10 for 12" braided stainless).
• Old washers reused on supply line connection: Hardened or deformed washers leak. Use fresh washers.
• Forgetting to turn off water before disconnecting supply line: Water sprays everywhere when the supply line is removed. Always shut off the valve first.

Niagara fill valve service interval

Typical Niagara fill valve service life: 8-15 years. The vacuum-assist Stealth models tend toward the longer end (10-15 years) because the lower flush volume (0.95 GPF) cycles less water through the fill valve. Hard-water areas with high mineral content shorten this to 5-10 years. Soft-water rural areas extend to 12-15+ years.

When you replace a fill valve, this is also a good opportunity to inspect and potentially replace:

• Flapper (often near end-of-life if fill valve is)
• Supply line (replace if 10+ years old)
• Shutoff valve under toilet (replace if 15+ years old or showing signs of corrosion)
• Tank-bowl gasket (if you notice any tank-bowl leak)

Where to buy

• Home Depot: Fluidmaster 400A at $10-$15. Stocked in every store.
• Lowes: Fluidmaster 400A at $10-$15.
• Amazon: Fluidmaster 400A at $9-$13.
• Ace Hardware / True Value: $12-$18.
• Plumbing supply: Fluidmaster 400CR pro-grade variant at $14-$18 — quieter operation than the standard 400A.

Alternative Niagara-compatible fill valves

• Fluidmaster 400A: Standard universal fill valve ($10-$15) — the recommended Niagara replacement.
• Fluidmaster 400CR Pro: Premium pro-grade with quieter operation ($14-$18).
• Korky 528MP Universal: Korky's multi-shank universal fill valve ($12-$16) — also Niagara-compatible.
• Korky 818-72BP QuietFill: Premium ultra-quiet variant ($16-$22) — for noise-sensitive bathroom installations.
• Fluidmaster Performax 400H: High-flow variant for older 1.6 GPF Niagara models ($14-$18).

For Niagara Stealth Dual-Flush owners

The Stealth Dual-Flush model uses a standard 2.5-inch shank fill valve (Fluidmaster 400A fits) — the dual-flush mechanism is in the flush-valve cartridge, NOT the fill valve. So fill valve replacement on Stealth Dual-Flush follows the same procedure as any other Niagara model. The dual-flush cartridge is a separate component (covered in the flapper page).