The master cylinder has a stepped bore (two different diameters, one for each circuit) The seals can fail internally, in such a way that they no longer hold pressure properly. This results in a soft (or worse) pedal, but no fluid loss. The fluid merely squishes back and forth between the two circuits. This is one of the more common failure modes. (happens a lot on Fiats too, about every 3-5 years or so)

Another thing I just thought of---it is possible for the bleeder screws to be obstructed, by corrosion or other debris. This is unlikely on new or rebuilt calipers, though I do recall once a fresh rebuilt caliper that simply WOULD NOT bleed, and when the screw was removed, it was found that the hole didn't go all the way through......, yes, that was on a Fiat too. Might be worth taking each bleeder screw out and checking it.