This might be your problem...
I had that happen to me awhile back on my 380 sl. Turns out the vaccum assist had blown out (big circular black thing that fits between the master cylinder and the firewall). I had virtually no brakes but it would stop if I pushed all the way to the floor.
There is a hose with a check valve that could be leaking or the cylinder itself.
Turn the car on, pump the brakes, if your pedal comes back up a bit your vacuum is working. If when you apply the brake, it slowly goes back down, you have a leak in the vaccum assist.
3 different brake places I went to missed it so you might as well check it out.