toilet takes a number of pulls before flushing (newbie)

My low level toilet is getting harder and harder to flush - having limited plumbing experience I wondered if there was anything simple that I could deal with or should I call in the professionals.
Thanks in advance
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more detail.
The crank at the other end of the flushing handle is connected to a rod that disappears into the top of the syphon. At the end of the rod there is a what looks like a piston with holes in it and on top of it there is a piece of flexible material which you need to replace. Cut a replacement out of a piece of heavy duty polythene and it will do nicely. You will obviously have to drain the systern and remove the syphon to do the job. Alex.
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Eat more spinach...
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