1973 Dart brake failiure

Mattax said
That looks like the correct master cylinder.

I don't agree. OP said the car has 4-wheel drum brakes. In their pic is a master cylinder for a disc/drum system. The correct master cylinder for the 4-wheel drum system has equal-size reservoirs, like this:

Mattax said
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.

The residual pressure valve was discontinued after the 1972 model year; drum brake wheel cylinder seals were redesigned to make the RP valve unnecessary.