How to fix a toilet that doesn't stop flushing - Never ending flush valve fix - Toilet keeps running
Straightforward fix you can try for a toilet that keeps running (the flush never stops). Simply add a bit of weight to the centre tube and you may find it fixes the problem without needing to take anything apart.
All you'll need is some way to add the weights to the centre valve; I've taken an old wire coat hanger and bent it into a hook that I can add some large metal washers to. Doing this has fixed the issue for us, saving us from needing to get a replacement, at least for now.. which is typically just before Christmas!