why is my cistern leaking from back nut of flush syphon?

went to change flooring, when lifted old flooring, found damp, wet boards. found drip from top of back nut and low porcelain/ceramic cistern base. changed flush syphon. Still leaked. realised not sitting in cistern correctly, undid all pipes, realigned, still leaked, tried sealant, left for 36hrs, still leaked, undid all, cleaned and dried everything, put syphon back in without connecting to flush pipe, just to see if it sat right, did up back nut, STILL LEAKED... HELP!!