Hi Sue
Conventional flush mechanism comprises a piston inside a cylinder which forms part of a syphon. Pushing flush handle lifts the piston which lifts a column of water up and over top of an upside down U bend to start the syphon which then empties the cistern into the bowl. The piston is normally a flat framework supporting a flexible plastic membrane which water pressure seals against the cylinder walls on the way up but relaxes allowing water to pass during its return. When the membrane wears or tears it requires progressively more force to operate successfully until it gives fails completely.
It's not difficult to replace the flap but some designs require you to remove the cistern from the bowl to do so. If you have this type its worth replacing it rather than just the flap - only a few pounds more but saves future trouble and expense in future. If possible get a flap shaped to fit your syphon rather than one you have to cut to shape.
Good luck with the job!
Bill May, March 2008
