Emilio Teodoro Lopez-Garazgos wrote:

Yes, I have tried filling the bowl slowly
with a pail. Same low level I'm afraid,

Try using a good closet auger. You might have a partial blockage that
is letting some of the water seep past sort of like how the bowl will
sometimes empty if it is mostly clogged.

I recently got a heavy duty one that was kind of pricey ($45 I think)
but that sucker is GREAT. It took about 15 seconds each to unclog my
ailing toilets (see a previous posting about that) and they've worked
fine since. It's 3' long but can be extended to 6'.