Usually when the diaphragm in the siphon unit fails people replace the whole siphon unit - but the diaphragm is just a piece of polythene - with a little bit of creativity you can make and fit your own diaphragm using - for instance - a garden compost bag - heavier gauge the better.
To get to the siphon on a close coupled system...
Turn off the water supply and disconnect the water feed to the cistern
Remove the lid and flush or bail out most of the water
Locate and loosen two wing nuts low down at the rear of the pan - they compress a squidgy washer.
Remove the two screws that hold the cistern to the wall
Lift the cistern upwards and tip any remaining water into the pan.
Disconnect the flush handle from the siphon
Undo the large nylon nut that holds the siphon in place
Remove and replace or repair the siphon or diaphragm
Put it all back together again - do not tighten the wall screws too much else you may damage the ceramic.
Good Luck...
September 2016
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