Short definition
A float-cup fill valve is the modern toilet fill valve — a vertical riser tube with a sliding cup-shaped float around it that rises with the water level and shuts the valve at setpoint. The Fluidmaster 400A is the canonical product (Fluidmaster’s marketing claims it’s the best-selling fill valve in the world). Replaced traditional brass-arm ballcock valves through the 1980s and is now standard on virtually all new toilets.
What it is
The valve body sits at the base of the tank, threaded through a hole and sealed by a spud nut on the underside. A vertical riser tube extends from the body up to about tank-rim height; on the riser, a doughnut-shaped cup floats. As water fills the tank, the cup rises with it. A linkage from the cup actuates a diaphragm at the base of the valve — closing the inlet when the cup reaches its setpoint.
Adjustment is simple: turn the screw on top of the riser to lower (clockwise) or raise (counterclockwise) the water level. Some models also let you slide the cup up or down by pinching its metal clip.
The Fluidmaster 400A is height-adjustable from about 9 to 14 inches, fits virtually all 1950s-and-later toilets, and includes integrated anti-siphon protection meeting UPC 603 backflow requirements. Korky 528MP is a comparable competitor. Fluidmaster 400H is a higher-flow variant for situations needing faster fill.
Why it matters to a homeowner
The standard cure for almost any old-toilet fill issue is a $12 Fluidmaster 400A and 15 minutes of work. Slow filling, won’t shut off, noisy whistle, leak from the tank-bottom shank — all four are usually faster and cheaper to fix by replacing the valve than by trying to diagnose and rebuild.
Procedure: shut off the angle stop, flush to drain the tank, sponge out the residual, disconnect the supply line at the tank-bottom shank, unscrew the spud nut from below, lift out the old valve. The new 400A drops in, you adjust the height, tighten the spud nut from below, reconnect supply, turn water back on, and adjust the water level with the screw on top.
When a plumber says “I’ll replace your fill valve,” that’s this part.
Common failure modes
- Sediment in cup or under diaphragm. Slow fill, noisy fill, won’t shut off. Pull the cap and rinse, or replace.
- Stuck cup on riser (mineral lockup). Replace.
- Worn diaphragm seal (after 10-plus years). Replace whole valve.
- Cup adjustment screw stripped. Replace.
Common variants
- Fluidmaster 400A — universal anti-siphon, current default.
- Fluidmaster 400H — high-performance / pressure-assist variant.
- Korky 528MP — comparable competitor.
- Older brass-shank Performax — same family, brass instead of plastic shank.