Had a callback on a multikwik push-button flush valve I'd installed about 18 months ago: it was letting by (water running into the pan) continuously. Found the black washer at the bottom of the drop-valve had gone soft (easy to get at: the mechanism unclips like a light bulb). I replaced it with the washer from a spare valve I had with me and later contacted multikwik who acknowledged that there's a problem with the material used for these washers in hard water areas, and sent out some replacements. The new washers are grey.