How to fix?

What is it? old, high lever, low level, or close coupled ie on the pan?
High and low level turn the water feed off bail out the cistern release the flush tube nut under cistern turn it down wards away from cistern. Undo the nut remaining under cistern this releases the syphon remove the syphon, Replace with new syphon I've never bothered with the membrane. Get a syphone that can be repaired without removing.

Close coupled again turn off water it requires removal of cistern from pan and its fixing screws ,if fitted, at back top inside cistern. Undo the syphon nut again replace with new syphon.
Can't say otherwise I can't see if the syphon is the repair in situ one. When buying syphon for this type get new doughnut rubber ring for seal between pan an cistern don't use the existing one.

charles
January 2009