In our tests of single- and dual-flush toilets ... As their name implies, these toilets have a flush valve that relies on gravity. Water drops from the tank into the bowl to move waste down ...

When air hits the siphon tube, it stops sucking in water. Most dual-flush toilets use a drop valve, which sits underwater at the bottom of the tank. Instead of relying on siphoning it uses ...

Dual-flush toilets usually have a mechanical valve with a rubber seal in the cistern that opens to allow water to flow when flushed. That rubber seal can rapidly deteriorate, says Tucker.