Emilio Teodoro Lopez-Garazgos wrote:

Relevant particulars:

*15 year-old American standard toilet
(not a 1.6 gallon flush)

*Ball-float is set so that water
level in tank is one millimetre below
top of filler tube

Problem:

After flushing, the bowl only fills itself
to about 2/3 of the depth that it used to
when new and with less "oomph" than
previously.

Manually flushing the toilet by dumping a
2 gallon pail of water directly into the
bowl yields the same (low) water level.

I had suspected than the holes in the rim
might be clogged with scale but that
wouldn't explain the low water levels when
pail-flushing would it ?

I'm wondering if perhaps a weir (or
whatever it might be called) somewhere in
the gooseneck of the trap might have
somehow gotten broken off, requiring
replacement of the toilet ?

The trap is cast right into the bowl and wouldn't break
in any way that didn't leak. Any sign of leakage?

What happens if you *slowly* add water to the bowl from a bucket?
If the bowl refills to its normal level, the problem is in the
refill mechanism in the tank. A small tube from the ballcock
feeds water down the overflow tube to refill the bowl. See if
that refillt ube is in place and that it produces a steady stream
down the overflow as the tank refills.

Jim