I have a concealed cisten that has just started over filling. I think it is due to the float not reacting quick enough to stop this. Im not sure of the model but it has a push button and the arm on the float is only about 3 inches long.
Either the rubber seal in the fill valve has perished (sometimes happens) and will need to be replaced - or - the valve needs to be adjusted - either lower the float further into the water or (more usually) turn the nylon adjusting screw inwards towards the valve - and do up the lock nut else it will need fixing again next week.
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