In article , "Peter
Ashby" says...
Feeling frustrated. Last night at 10:05 the toilet siphon broke
unreservedly. So, the wife dropped me at B&Q to pick up a new one this
morning and I walked home. So far so good. Because our cistern sits
directly on the toilet there is no connecting pipe so I have to remove
the cistern to get old siphone off. So I go up into the attic and turn
the gate valve on the pipe emananting from the cold tank, come back down
check sink tap, water gushing, blast. Go up and down fiddling with it,
tracing pipes, reconfirming the system is how I thought. Conclude valve
is bust.

Stick a carrot in the pipe inside the tank.