I am having a little trouble in sorting out the WC cistern in my daughter's bathroom. The plumbing is approximately ten years old and the mechanism within the WC cistern is all plastic. The water was up to the level of the vertical overflow pipe and was dripping to the outside. Adjustment of the plastic piston failed. I have replaced the rubber diaphragm and rubber 'O' ring, within the plastic cylinder, but it still drips after a while. On the plastic float arm there are six holes, each the size roughly of a five pence piece. In the fourth hole from the float is a red plastic disc, secured by a plastic split pin. What is this for, please and can it be moved along the arm? If so why? Failing this any ideas would very much be appreciated. Many thanks.