How to stop toilet running on , coroma duel flush 1995/96?

I have a coroma duel flush toilet , 1995 model , the seals are fine in it and they work well however when you flush the toilet the water keeps running through the bottom of the cistern , the only way of stopping this is to wriggle and tap on of the buttons at the top then it will stop and fill up properly , i have looked at everything possible and pulled it completely to bits , all the parts are plastic and lift out from a cage with the four locking tabs,everything works free in the mechanism when out of the cistern or with no water , the lever action works perfect , but when the cistern is full and flushed the seal shaft hangs up just slightly to keep letting water run out the bottom and you cant see why its sticking there.