I pretty much have the same problem. Except I do have fluid at the left rear wheel cylinder and fluid between the master and booster. So I found mine. But I had to pull the wheel and drum off to actually see it. I also pulled the master apart from the booster. And there was fluid between them.
I was going to replace the rear shoes, not until I get a new wheel cylinder. Don't want to contaminate the new ones.
Should I replace both sides? Or if it ain't broke don't mess with it.

 

Personally, I'd do both including new cylinders. If one is leaking the other one may not be far behind. If you do 1 set of shoes you may get different braking on the rear, at least in the. These trucks are bad enough with locking up the rear brakes.

Wonder how 78fordtruck78 made out? Nice to know what the problem was so the next person can learn from it.

 

I"d say Master cylinder I had the same issue on my 79.You will see no leaks its bypassing internally.

 

I changed both wheel cylinders and both ( left and right rear) shoes. My brakes are great and I have an E brake again.
My master had fluid between it and the booster so that will next hopefully befor I have to do it.
Just wanted to thank everyone that posted replies and the OP. just so y'all know this helped me.