May only need a washer, but siphon will have to come out the cistern to fit it. Personally, if removing the siphon, I'd opt to replace with a modern, 2 part siphon, to allow for easier servicing in future. Also could be a possibility the existing is split slightly, and any air leaks will effect it's operation.
Be careful disturbing the flush pipe though if that is also the original lead.
1. I recently replaced one of these at home - an original from 1936.
2. The inverted "U" part - at least the verticals were made of diecast zinc alloy, and were pin-holed in places. They could not realistically be fixed.
3. I just replaced it with a modern one - a Dudley 88 turbo adjustable from memory.
4. As Hugh Jaleak says, be very careful separating the syphon from the flush pipe.
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