On Friday, October 2, 2015 at 2:26:52 AM UTC-7, John Smith wrote:
On 02/10/2015 11:20, micky wrote:

Both toilets are more than 10 years old. I understand that the flapper
probably has passed its useful life. I just don't understand why it did
not leak before cleaning but did so after cleaning.

Toothbrush is quite soft. The other brush is a bit harder. Yet, there is
no scratch marks on the rubber. The rubber has some "goose bumps". I can
only guess that gunk filled up the gap between "goose bumps". After
cleaning, the gunk is gone and the flapper starts to leak.

A toothbrush isn't enough. You need to scrub both the flapper and
the seat with a wet cloth or paper towel.

Goose bumps indicate the flapper was made of vinyl, or maybe a blend
of vinyl and rubber. They'll disappear after the flapper dries out
for a few weeks, and some vinyl flappers will also change from grey
back to black. But a vinyl flapper can develop a permanent set,
like a ring indentation where it pressed against the seat of the
drain, or even a warp, and it's common for them to not seal any
more.