Assuming the water level in the cistern hasn't changed (you can tell by the water mark on the cistern) you need a new syphon kit - you should be able to get one fairly easily from a DIY store or bathroom centre. If the water level is up high, the ballvalve is letting by - it needs re-washering or replacing. Regards Colin Evans PBSL Group
i always replace the whole flush valve with the factory one....not just the flapper...the whole thing...i remove the tank and unscrew the whole flush valve....
it works 100% of the time...
just replacing a flapper can have mixed results...there could be some damage on the rim of the flush valve and no new flapper will work....
i do stock the wolverine brass 'big orange' which is supposed to be the best....i think i used one once.
That model of toilet doesn't have a syphon but rather a dump valve.
But anyway, the fault most likely is the rubber seal is faulty or has grit trapped in it.
It can be accessed without removing the whole valve. With the water supply off, the top of the inlet valve can be removed by turning 1/4 turn anticlockwise in the same way as a bayonet light bulb. Beneath is a black washer the size of a £2 coin. After taking note of which way up it fits, prise it out and clean/replace with new.
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