Brake pedal goes to floor

[70 shorty]

I have a 70 4 wheel manual drum truck, and the pedal goes to the floor. Upon investigation, the back of the master cylinder is empty, and the front is down a little, (although the front may have been what spilled when I took the cap off). My question is, does the back of the master cylinder feed the back brakes, or is it separated? I see the line from the back of the master goes into the distribution block, but does it stay separate from the front? I pulled both back drums and didn't see any obvious leaks on the wheel cylinders. Since the truck sits a lot though, could it be possible that if a wheel cylinder was leaking, the fluid could have dried up? If nothing else, I'll dump more fluid in the master, and see if I can see where it's leaking, but I figured I'd start here, and see if anyone had any tips.