Probably MC but check vacuum booster.
Had the same problem with my 380sl. The pedal was firm at startup and a couple of stops. Usually after 2 or 3 stops, the pedal would go all the way to the floor. I could hear a whooshing sound under the dash. If you can still get braking if you push all the way down, I'd bet it's the vaccuum booster (behind the master cylinder). I learned the hard way by having the MC replaced, only to drive off and have no brakes after spending $150 for a new MC I didn't need.
Vacuum boosters are cheap to buy (used less than $50) but a pain the the butt to install so I hope for your sake it's the MC. Check the manifold vacuum hose first.