Ball valve types
There are basically two main types of float valve, those with a solid metal arm and those with a plastic arm. They are sometimes referred to as a ballcock.
When correctly adjusted, the valve should close off the water feed a little before the level reaches the overflow outlet.
Cisterns which continually overflow can usually be cured by a simple adjustment of this arm.
Adjusting metal arm ball valves
For metal arms, hold the end nearest the valve with one hand and, using the other hand, bend the end nearest the ball float. Bend it down gently to lower the level at which the water flow stops. Bend it up to increase the level.
Adjusting plastic arm ball valves
The plastic types are controlled by a screw stop mechanism. Release the stop nut, and screw the threaded section out to increase the level or in to decrease it. Once the correct position has been found, reposition the stop nut.
If the float arm requires a lot of upward force to stop the water, it is a sure sign that the washer has become worn and needs replacing.