Friend of mine said he had to fix an old Kohler toilet that leaked at the base, so he was going to change the wax seal. I said that I did not think you could get a leak at the flange unless water was backing up from a clogged waste line, and that the purpose of a wax seal was primarily to prevent escape of sewer gas into the room.

He had Mike Diamond Plumbing check out the waste line for their advertised $99 price, and they pronounced it fine on basis of turning on faucets and observing no backing up. They told him that "those old Kohlers flush hard and that makes them leak past the wax seal", then they offered to re-seat the toilet for $340.

How could a "hard flush" from a 2" diameter toilet trapway push water back up past the flange as it drops into a 4" waste line?