Hi, I have a dual flush toilet that's leaking water from the cistern to the bowl. I've taken the flush system out cleaned the washer of gunk, cleaned the bottom of the tank and it's still leaking-in fact it was leaking more at 1st! I rotated the unit a little and it slowed down; I can't rotate it more though. Do you think the washer needs replacing? I've checked it, it looks tight ton the unit, cannot see any cracks or splits on it. It is 15+ years old though.... Any advice gratefully accepted.