Brake master cylinder

I have a 1985 Chevrolet 3/4 ton truck that has lost the fluid from the smaller, rearmost reservoir of the master cylinder. Am I correct in thinking that this is for the rear brakes? Or is it for the front ones?

The rear reservoir is for the front brakes.

I’ll bet if you remove the brake master cylinder from the brake booster, you’ll see brake fluid leaking into the brake booster. Which means the rear seal in the brake master cylinder is leaking.