cistern leaks into bowl Caroma mini uniset approx 15yrs old?

Have replaced ball float many years ago with Fluidstyle unit works great only recently does the cistern leak into bowl, causing unit to refill every 30 mins or so, have adjusted water height so as below the sq tube in centre but still overflows poss through bottom outlet allowing water to leak into bowl not sure how to take out centre unit or to replace seals if any there