On a toughie (& this one sounds (very) close!) I cap the MC with a pair of brass plugs from the Edelman cabinet at your parts house (1/2-18 or 1/2-20 or 9/16-18 or 9/16-20 I cant remember which) & if the MC is good (& bled out) the pedal will be rock solid with virtually no travel then I hookup the frontmost port for the rear brakes & see it is still solid so far. What I would do on this one is unscrew the next to the firewall MC line for the front discs & cap that MC port & see if they (the rest of em) are solid.