On a toughie I would cap the MC ports with a pair of brass inverted flare plugs from the Edelman fittings cabinet at your parts house. then with eng idling if the MC is holding psi (it s working good) & the MC is bled out adequately and the booster/check valve is functioning normal the pedal will be rock hard with virtually no travel. this'll tell us where to start. Discs can be hard to bleed out on occaision