Many plumbers' merchants still stock diaphragms for syphon-type cisterns (although they are being phased out in favour of flapper-valve types). The diaphragms are usually sold as a set of assorted pieces. I have made them out of a piece of fertilizer bag successfully. The size is critical, and the edge should be free of nicks for a good service life. You will need to dismantle the cistern first to discover the size and shape of the diaphragm.
Tony, March 2009
