1973 Dart brake failiure

The next steps are to
(a) check for leaks, especially the hoses.
(b) start prepping the flare nuts for removal.
(c) Get the tools and/or experienced person to help with breaking the nuts free without damaging the tubes. Otherwise you will be replacing the tubes. And compression fittings are NOT an acceptable repair of brake lines.

The reasons the rear braking was largely ineffective with the front brake resevoir empty has much to do with how the master works. See page 1 of the Brake Hydaulics MTSC linked earlier.