[cdvs]

If you have to 'stand' on them to work, it could be the booster, or vacumn lines to it. But, the booster just makes it easier, not better.

If you push on the pedal and it slowly drifts to the floor, it's the master cylinder leaking internally.

If neither of these applies, it could be:

A pinched or otherwise blocked brake line.

Air in the lines. (even ones you didn't detach)

Caliper not sliding on bracket.

Good luck!