fowler twin flush cistern?

Make sure all the fittings going through the cistern have a rubber seal on the 'inside' of the cistern. That includes Siphon, water inlet and overflow.
Do not over tighten, hand tight and then a gentle wrench turn to that which feels secure but not over tight. (Plastic is a delicate material especially on the threads)
If there is a larger rubber seal to fit between the cistern and pan, then mould it to fit securely on to the siphon thread and over the siphon backnut going through the cistern, before placing it on the pan. Ptfe thread tape is another good tool to use in securing threads and fittings in conjunction with rubber seals. Silicone too (but only as a last resort.)
If the water is running continuously in to the pan then you probably have a defective siphon.

Thread jumper
December 2010