You're thinking of Duo-Servo rear brakes. Most of the 1/2 ton trucks use Twin Leading Shoe brakes, which need the park brake to adjust.Drum brakes WILL self-adjust. but only after they get fairly loose, and the self-adjuster mechanism is set up so that it only adjusts when going in reverse.
Prejudice? Maybe. I really hate Twin Leading Shoe brakes.
Yep.its different than what you think, mine doesnt have a slot in the backing plate to get the adjuster and its not reverse on mine, it works off the parking brake