It is likely that the polythene diaphragm in the siphon unit has
disintegrated, resulting in lack of sufficient suction to siphon water down
to the toilet bowl. You may be able to replace the diaphragm if you can find
one to fit. To do that you will need to remove the siphon unit after turning
the water supply to the cistern off and draining the cistern of water. If
you can't or don't want to replace the diaphragm you can just replace the
whole siphon unit. A new unit is only about a fiver. Take the old one with
you to get a similar size/type of siphon unit. New siphon units can leak
from the nut/thread under the cistern; I use clear silicone sealant to
. You may be able to replace the diaphragm if you can find
A tip from this group when I had the same problem was to remove diaphragm
and replace with a piece cut from one of those clear (usually coloured)
plastic document wallets/folders. I did this with two toilets and they are
working as new.
My units were designed so that you could get access to remove the syphon
easily, without having to do anything too techical. After that it was dead
easy to replace the diaphragm as above.
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