It's not uncommon on more complex brake master cylinders to trap air in the master cylinder itself. It's often suggested to bleed the master cylinder on the bench before installing. In lieu of that have a friend press the pedal while you crack the brake line fittings, one at a time, at the master cylinder.
If you find that you've solved the falling pedal by doing this, check to see what happens if you press hard on the pedal for sometime. If it gradually falls, you've still air in the master cyylinder somewhere and may need to do a pressure bleed with one of those master cylinder attachements.