I am not an expert but, i have the same kind of push button flush and mine used to do the same, but if you look at where the water is flowing back in to fill up again, you will see the ballcock arm moving back in to stop the water flow when it reaches a certain level. At the end of this arm inside the mechanism is a round plastic housing which has a little black rubber disc about the size of a twenty pence piece which sits in the housing. This is the little blighter that stops the water through pressure suction,but sometimes this little blighter develops a little pin prick hole therefore causing the suction to not work, so this will need replacing and cost about £2 to £3 and can be bought at your local bathroom hardware store. You need to take the rubber disc with you as it is easier if they see it and will know which one to give you! You can take this rubber disc out yourself if you are a bit of a d.i.y. person as you don't have to take everything to pieces, just the housing where the arm goes into as it is just one plastic screw ring at the front of the mechanism housing where the arm goes in, but just take your time and watch closely how everything comes out so you know how it will go back in!!  I hope you can understand this if your system is the same as mine as it may save you a few quid of calling in a plumber that can charge you a small fortune. Hope this helps! Good luck.