WC valve flushing devices have been KNOWN to leak for 160 years. The siphon can NEVER leak.
wc flushing devices
WC valve flushing devices, either flap valve or drop valve, are just like a "plug in a sink", with water ALWAYS above the valve seal. When the seal fails, water will leak continuously until the valve flushing device is repaired or replaced.
leakage graph
deformed flapper valve
Valve flushing devices were outlawed in Britain in 1861, after Thomas Crapper and others introduced ‘The water-waste preventer’ or common siphon as we know it, but made legal again in 1999. Materials may have changed but the laws of physics have not.
bullet point ALL UK toilets with "Push-buttons" have VALVE FLUSHING DEVICES, which CAN LEAK.
bullet point ALL UK toilets with "Flush-handles" have SIPHONS, which NEVER LEAK.
Valve flushing devices have undergone 150 years of development in America and they still leak. The siphon can never leak.
At any one time in America, one in five valve flushing toilets leak at a rate of 20,000 gallons per year. Adding all the lesser leaking toilets to this and text books equate this to 15 to 30 litres per day for every person in America. (Population 250million). See pdf file for source.
Valve manufacturers do not deny the fact that their devices leak. When asked the question, ‘but don’t these valve flushing devices leak?’, a chief executive of one of the largest valve manufacturers in America, replied, ‘of course they leak, that’s why we sell so many’.
How then, is the valve displacing the siphon, everywhere?
  First, because the siphon has been largely maintenance free and totally leak free, people have wrongly assumed that any other device in the cistern will offer the same ‘fit and forget’ benefits. Such is the faith in the siphon that ‘boxed in’ cisterns have been a feature of bathroom design for some time now.
Second, because people believe that whatever device is in the cistern, ‘it will be alright’; people choose a toilet entirely on its cosmetic appearance. Hence toilets with ‘simple push button control’ or even ‘no touch’ sensor control are ‘all the rage’; people can’t get enough of the ‘latest technology’, a salesman’s dream. People don’t even bother to ask what is inside the cistern.
Third, once a valve toilet has been purchased, a siphon cannot later be fitted. The outlet diameter is different and there is no hole in the cistern to accommodate a flush handle. This is not an accident of design. The hapless purchaser finds himself locked in to buying ‘repair kits’ (as intended by the valve manufacturers) or worse still; just putting up with a toilet, which constantly leaks water and money down the drain.
So, a combination of slick marketing by powerful companies and the blissful ignorance of consumers, has led to the displacement of the siphon by the valve, worldwide. Even British siphon manufacturers now make valve-flushing devices; such is the strength of misguided consumer demand.
For an independent detailed analysis download the pdf file ‘WCs; best practice since the Water Fittings Regulations 1999’, from elemental solutions, which was written before the interflush was conceived and before interruptible flushing was legalised. See also, ‘Water-efficient WCs and retrofits’ pdf file.