push flush cistern?

Replace your fill valve, Depending on the brand of your cistern (toilet) and the location. I assume you are not in the U.S.A?
90% of the toilets here use a fill valve mfg by Fluidmaster. the top cap can be poped off and then a simple 1/4 turn twist will expose the rubber gasket. go to this website it shows you an example of how to fix your issue without replacing the entire fill valve. http://www.danco.com/html/Toilets_Frameset.htm they will show you step by step how to fix and repair your unit.

Marc E.
February 2006