Thought I’d throw this out there for consideration. I suddenly had my brake pedal go soft while driving. Pulled over and checked the front reservoir found it almost empty. Checked all the fittings, could find no leaks. Got it home and started really looking. On my 1978, the metal line that services the front left caliper passes under the inner fender drivers side. Apparently, over time the abrasive action actually cut into the metal line causing it to fail. Replaced with the new style copper(can be bent by hand) which is DOPE. Rerouted it away from the inner fender. Up and running.
Weird thing is it’s factory original. Line & location. Could be over time the inner fender dropped a little causing the wear. I relocated the new line away from the fender.
Oh wow. I noticed my hard lines were really close to touching that same spot too. Gonna learn from your situation and bend the inner fender back a little bit now.