hi john,
it sounds like the diaphragm has gone in the siphon, unfortunatly it is not repairable and the whole siphon will need to be replaced, but dont worry a new siphon will cost you between £5 - £10 and is literally a 5 minute job, just remember to turn the water off first, remove the old siphon and the new one just goes in exactly the same way that the old one came out. turn the water back on and "hey presto"
scott
December 2007