1973 Dart brake failiure

That looks like the correct master cylinder. Fix it. Correct replacements are almost only available as new old replacement stock (NORS) and last I checked were going for over $200. You can throw a 'cheap' replacement on temporarily if you are short of cash or in a rush. These are made by removing the residual valve from the outlet for front brakes of a drum brake master cylinder. Several threads about this.