Short definition
A ball float is the hollow brass or plastic ball on a long arm in an older toilet tank. As the tank fills, the ball rises, the arm rotates, and linkage closes the fill valve. Standard tank-fill mechanism for most of the 20th century until the cup-float Fluidmaster 400A took over in the 1980s. Still common in pre-1990 WA homes.
What it is
The ball is 5 to 7 cm (2 to 3 inches) in diameter, hollow, and floats on the rising tank water. It’s mounted at the end of a 6 to 10-inch arm that hinges at the fill-valve body. Inside the fill valve, the rising arm pushes a plunger or diaphragm against the inlet seat, shutting off water when the tank is full.
Adjusting the water level on a ball-float toilet is usually as simple as gently bending the brass arm — bend down to raise the level, up to lower it.
Why it matters to a homeowner
The ball float’s classic failure is waterlogging: a pinhole in the brass or plastic shell lets water in. The ball gradually fills, sinks lower in the tank, and eventually fails to rise enough to close the fill valve — so the toilet runs continuously. The diagnostic is straightforward: lift the arm, shake the ball, and listen for water sloshing inside. Water inside means replace the ball.
Replacement balls cost $5 to $15. A whole ballcock assembly is $15 to $40. The standard upgrade — and what most plumbers recommend — is converting to a Fluidmaster 400A cup-float fill valve for $12 to $25, which is more reliable, quieter, and easier to adjust.
Common failure modes
- Waterlogged ball. Top failure mode. Replace, or upgrade to cup float.
- Bent arm. Water level off; bend back to spec.
- Sticky linkage in fill valve. Clean or replace.
- Cracked ball. Replace.
Common variants
- Ball float (this entry) vs. cup float. Two mechanisms, same job.
- Ball float vs. flapper. Flapper is the flush-valve seal; ball float is the fill-valve sensor.