1973 Dart brake failiure

Pop the top off of the master cylinder and visually inspect for fluid, or lack of.
Slip the spring clip off to on side or the other, the lift the lid.
If there is fluid in the reservoir, and no leak visible anywhere from any of the wheels, steel lines, or rubber hoses, the seals inside the master are probably worn out and no longer holding pressure.
It's possible that your master cylinder has a single bolt holding the lid down as well, I'm not positive when the change was made, if so it will be a 1/2 inch or 7/16 bolt head.