Did you take the rear drums off, open the 2 dust covers on each wheel cylinder and look for seepage?
Since you already spotting previous owner had swapped the lines going into the master cylinder incorrectly....AND it only had single flare lines to it... you need to start from scratch.
Trust nothing was installed correctly.
-Verify every part in the brake system
-note was is stock
-what is aftermarket
-what looks 50 years original
