1973 Dart brake failiure

I don't agree. OP said the car has 4-wheel drum brakes.
You're right he did say that but the photo shows a disk brake master. So I could be wrong or I could be right. LOL
I think we agree its best to use the correct master cylinder for the brake system, which ever it is.
Yes his car is a '73 - for some reason I was thinking it was a '70 in spite of the subject line. the brain is weird thing.