How do I isolate the leak?

Have you pulled the drums and inspected the rear brakes? Here is a list of things to look for:
1. Stuck wheel cylinders. This happens when corrosion locks the pistons in the bores. Wheel cylinders are about $10 each and change easily unless brake line fitting is rusty.
2. Brake lining detached from brake shoe. Replace
3. Adjuster cable rusted, stretched, or broken. Replace.
4. Adjuster locked with rust. Spinning the star wheel is not possible. Replace.
5. Brakes are not adjusted and centered, such that there is just a bit of friction as wheel is rotated. Adjustments are made by pulling the rubber plug and rotating the star wheel to move the shoes out. Press brake to center, adjust ... If too tight it is difficult to go the other way, since self adjuster stops the star wheel.
6. Consult manual. Check if shoes are in incorrect locations. Check the springs, star wheel, and self adjuster are correctly installed. Things are similar front to back, but vary from side to side. Someone may have been in there before you and made mistakes.