Is the rubber hose above differential new ? If there is a hose or fitting allowing air to enter, standing pressure will reveal it although bleeding procedure may not.
Years ago we were working on a Pontiac called The Judge ( I forget year model ). Anyway, that car sat with a pressure bleeder hooked up for more than half the work day before a tiny hair line crack in a caliper casting was revealed.
That sounds as if air would go in a lot easier than fluid would come out, but actually air was going both in and out. Only the pressure bleeder eventually pushed fluid out.