Two of my toilets are of the same model. Both are made by American
Standard and both use the flapper as shown in the following link:

http://www.lowes.com/pd_236807-143-P...ductId=3375098

Lowe's sells this flapper for around $9. Here, I need to pay more than
$30 for it.

About three months ago, one flapper would not come down after flushing.
I thoroughly cleaned it and then put it back. It started to leak. I
tried many things but ended up buying a new one.

Now, the other toilet has the exact same problem. Yesterday, I was
informed that the flapper would not come down. I went to check it. It
was not leaking at that time. I removed the flapper and cleaned it.
After I put it back, it started to leak, just like the other one. I used
a toothbrush and a plastic brush to clean. They should not damage the
flapper.

Can anyone explain why a flapper which was not leaking leaks after
cleaning? Much appreciated.