The ratchet affects both the parking brake and the regular brakes on the '91. The ratchet controls the distance from the brake shoe to the drum. As the brakes wear, it slowly moves the shoes out so you don't get increased pedal travel. If your regular brake pedal (Not the parking brake) seems to have normal travel, there is an adjustment to the parking brake cable under the left side of the vehicle on the outside of the frame. In the cable you will find what is called a cable equalizer. It is a threaded rod that runs through a bracket and has a spring attached. By tightening down the nut, you put more tension into the parking brake cable, which will cause the parking brake to engage sooner. According to the manual, for a '91 or '92, to adjust for cable stretch (good probability in a '91), you first fully apply the parking brake then turn the nut 6 turns. Release the brake and check for rear wheel drag (There shouldn't be any). If the brakes are still loose, continue doing this until the parking brake applies enough pressure to hold the vehicle, but does not drag when the brake is released.
If your regular brake pedal does have too much travel and your fluid level is right, backing the vehicle up and slamming on the brakes a couple of times can adjust the ratchets