Need some electrical help with my ‘68 Plymouth Valiant 100

Do you see any streaking on the inside of the rear backing plates? Look under the car on the inside of the brakes and tires.
Drum brakes. I checked the brake fluid and the front half of the master cylinder os completely gone. So most likely something is wrong with the rear drums, or at least that’s what I assumed. I filled it with brake fluid and tried pumping the brakes to see if I could find leaking from the rears, but I couldn’t seem to find the leak