Drum brakes need to keep the shoes close to the drum even as the lining wears away. The part that handles this automatically is the self-adjuster. Here is what a brake self-adjuster does.
Why adjustment is needed
As drum brake shoes wear, the gap between the shoe and the drum grows. If nothing corrected it, the pedal would sink lower and lower as the shoes thinned - see how drum brakes work. The self-adjuster takes up that slack so the pedal stays firm.
How it works
The self-adjuster is usually a threaded star wheel with a lever that clicks it a notch tighter as the shoes wear, often triggered when you brake in reverse or apply the parking brake. Each small adjustment keeps the shoes the correct distance from the drum.
When it seizes
Self-adjusters are exposed to heat and moisture and can rust solid. A seized adjuster stops taking up slack, so the brake pedal travels farther and the rear brakes feel weak - and one side can end up adjusted differently, causing an uneven pull; see uneven braking.
Service it
When doing a drum brake job, clean, lubricate, and free the self-adjuster, or replace it in the hardware kit - see brake hardware. Fresh shoes and working adjusters keep the rear brakes firm. Keep the front discs sharp with quality pads and hardware.