Chris Bacon wrote:
John Stumbles wrote:

From your description it sounds more like a flap valve


These things are, as they say, pants. Water wasters.

Not tried the fluidmasters or other flap valves. I agree drop/flap
valves have a failure mode in which they can let by whereas syphons'
only failure mode is simply not flushing, so in that respect they're A
Bad Thing (and presumably why they were resisted by the Powers That Be
until recently). It's a pity no-one makes conventional syphon valves
just a bit shorter than standard so they can be used to convert a
standard cistern to internal overflow.