So the smaller compartment is fluid for the rear brakes. They have two separate chambers so it it isolates the front from the rear providing some redundancy in case of failure.
So if you have no fluid in the small chamber, it means you probably have a leak. Check both wheel cylinders in the rear for leaks. You may have to replace them and then bleed the brakes once you fill the fluid.
1992 Black YJ, 4L I6, 8.8 Rear
3.5" BDS, 5/8" MORE Shackles,
DT-3000s, 16x8 Bajas,
285/75R16 Dunlop Mud Rovers