I have replaced three toilet bowls and cisterns over the last 7 years. I had to remove the bathroom one because of a leak on the inlet side (don't ask why that is a much longer story). When I have replaced it temporarily I noticed that the outlet where it joins the plastic waste pipe was leaking, very slightly. I checked that the rubber seal was in good condition and that it was fitted properly - seems to be - but still this minimal leak. I have always been dubious that just pushing it on did not seem particularly secure but figured that was the way it was. The connecting pipe has a good fall. The only thing I can think of is that the point where the porcelain meets plastic and rubber was/is fouled after some years usage and maybe that has caused the rubber to not give the necessary seal. Or should I have put some additional sealant at that joint? Or any other ideas? Cheers.