My dual flow toilet runs constantly into bowl?

My duel flow toilet flushes fine. But afterwards, a small amount constantly leaks out into the bowl, so every thirty mins or so the toilet inlet 'tops up' the cistern. I have taken it apart and removed the large cylinder shaped device and cleaned the large rubber washer at the bottom (only 18 months old). I have put it all back together fine, but the problem is still the same. At least I did not make it worse!!
Please help.
It is a dual flush top mounted button device.

Jon