I replaced the siphon unit in a close coupled toilet at the week-end because it had not been flushing. The toilet is about 15 years old and the metal plate through which the cistern is connected to the toilet (by wing nuts) was badly rusted and the various seals were in poor condition. However there was no problem with leaks or water escaping previously. Now when i flush the toilet a significant quantity of water (say 15%) spills out the between the cistern and the toilet. I suspect that the seal between the siphon unit and the cistern is not tight enough, but there is no leak apart from when flushing. Is the solution to simply tighten the nut more (It is already as tight as i can tighten it by hand)? Would it help to insert a seal between the nut and the base of the cistern? I have not replaced the metal plate as i felt it was preventing me from tightening the nut fully. The cistern is very stable.