How Does a Dual Flush Toilet Valve Work? This is the Best Explanation.
A dual flush valve is slightly more complex than the single flush unit but works on the same principle. Instead of one float there are two.
Modern dual flush valves are usually cylindrical in shape. The full-flush float is positioned at the bottom of the cylinder, as with a single flush valve, and is operated by the full flush button positioned above on the cistern lid. The half-flush float is positioned half way up the valve body and is operated by the half flush button above. Both the full-flush float and the half-flush float are connected to the seal below. They are not however connected to each other and lift the seal independently of each other. Refer to the diagram for the details.
When the full-flush button is pushed down, it raises the float at the bottom of the cistern. This float is connected to the seal which lifts and remains open until the water has drained down to the toilet below. Once the cistern empties, the float drops. This causes the seal to drop shut and the incoming water again fills the cistern (as with the single flush valve described above).
When the half-flush button is pushed down, the half-flush float is raised. This float is also connected to the seal which is raised and remains open until the water drains below the level of the half flush float. Once this float is exposed by the falling water, it drops, returning the seal to its closed position. The inlet valve then fills the cistern to the required level.