Your issue sounds like a bad brake booster. A leaking brake master cylinder should result in a soft/spongy brake pedal. Your brake booster uses vacuum to amplify your brake pedal pressure to assist you when braking. If the brake booster or check valve fails, the pedal become hard because the vacuum is not assisting you.
How did you check the booster?
Here are the vacuum tests required to verify brake booster and check valve operation:
Booster Check Valve Test
Please let me know if there is anything else I can do to assist you.