This does sound like an early type of siphonic pan. Syphonic pans have two traps (water seals) with an air space between them. When the cistern is flushed water travels down the flush pipe and as it does sucks air out of the air space causing a slight vacuum. When this vacuum is created the pan's contents disharge. If there is an air leak between the traps this vacuum cannot be created. It may be a simple matter of resealing the small tube to stop any air being drawn into the system other than from the trap air space.
BarryF
December 2009
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