On Thu, 11 Apr 2013 23:42:52 -0700 Oren wrote:

Pour a little more acid in the bowl and brush the bowl edges with the
nylon toilet brush - brush UNDER the rim also.

This is good advice as the experiment was done for the acid,
so there was no brushing yet ... just chemicals ... on purpose ...
so that we could tell which worked best
bleach -- useless for the brown stains
naval jelly -- nearly useless for the brown stains
pool acid -- perfect for the brown stains

Does the toilet flush with a strong vortex (swirl) and siphon?

Yes. But I noticed "other" toilets in her house which seem to
need "the cootie treatment", so, I told her I'd come back to
finish them off.

Any other ideas for experiments?