I have read some older posts concerning this but have a few specific queries. I have old Kohler toilets (about 25 years) with overflow through 5 largish holes at the back of the cistern --- the original idea being to drain through a floor drain. However uneven floor makes this impossible and anyway I think the drain is blocked. I have read about internal overflow syphons and they seem to be the answer with the holes being blocked up. One reply talked about detaching cistern and laying it in bath to change the syphon but this is not poss with mine as it is all in one with toilet bowl. Any comments and suggestions would be much appreciated