I would bet my last dollar that it is (as said) the rounded booster rod nub clearance. Use a valvetrain pushrod to measure the distance from the rear MC flat flange to the depth of the piston "thimble" then adj the booster rod protrusion out from the booster flat mounting flange to be .020" less than that. Still bad, then take off the MC lines & cap the ports with some brass plug fittings (be sure to get the right thread(s) or grab some junk fittings/lines & mash em shut close to the fittings. eng idling, with the MC capped the pedal should be rock hard with virtually no travel. holler back. EDIT this is with the MC bled out which I am assuming you have already done as a first (seperate) step in the bleeding process.