In article ,
"Andrew Mawson" writes:
There is usually a polythene diaphragm that forms the seal for the
piston that starts the syphon off, and after a while it wears away. If
you turn off the water, do a sucessful flush to empty the tank, and
remove the gubbins all will be revealed. My local (rather good)
plumbers merchant stock them and they cost (last time I bought one)
the princely sum of ten pence ! As I remember there are two sizes &
shapes but it is only very thin polythene sheet so at a pinch you
could cut one out with a craft knife.

If it's an old loo, it might be a rubber diaphragm. These can perish
after a long time, and either bits break off, or it stops bending
out of the way when the flush starts off and throttles it.

Another possibility is an air leak into the syphon. It could have
broken somewhere, or the plunger hole might have worn too large
(although modern ones capture water around it to maintain the
air-tight seal).

If it had never worked right, I might also suggest that the cistern
and pan had a mismatched capacity, e.g. a 6 litre cistern trying
to flush a 10 litre pan. However, given it did work at one point,
this can probably be ruled out.

--
Andrew Gabriel