At your parts house in the "Edelman" brass fittings cabinet get a pair of inverted flare straight thread plugs to cap the M/C ports. On the end they will have a recessed flare a mirror image of the brass seat(s)in the M/C. Cap the M/C & if it's good (& bled out) the pedal w b rock hard w virtually no travel. (1)Jump on the pedal hard and (2) put steady light pressure on it, these 2 tests will tell you if it is the prob or look further downstream. Or if not wanting to grab some fittings rebleed the M/C as it sounds like it's bad or there's air in it