If the drum brakes have to function correctly, the brake pads have to remain close to the drum. If they have a considerable gap between them, extra force is needed to make them apply with proper friction against the drum and this would require more fluid to pass through the pipes. This also makes the driver to push the brake pedal to its maximum for good braking.
Why does this happen?
This happens because the brake shoes wear down due to usage over time. So the brake pads have to be brought closer to the drum again for proper functioning. Some brakes require this to be done manually, where others have a mechanism which does it automatically.
Parts of an auto adjust:
Parts of an auto adjust includes an adjust lever and an adjuster gear. The adjust lever is attached to one of the brake pads and the lever is free to move around the fixture. The adjuster gear is fixed on the other brake pad. It has threads on its to enable tightening of the brake pads and has a screw surface for the adjust lever to lock on.
Lets imagine a condition in which the brake pads have worn considerably and the gap between them and the drum is enough to affect braking. Now, whenever the car stops in reverse, the brake pads are pulled away and the adjust lever, due to its position, also stretches according to the gap between the pads and the drum. When the brake is released, the adjust lever locks on one of the screw threads and rotates it as a result of the withdrawing motion. This extends the screw a little more and it holds the brake pads further away and closer to the drum.
Disadvantage of automatic adjustment:
There is an entirely different kind of adjustment seen in disc brakes. According to the braking mechanism in disc brakes, they don’t require an auto adjustment mechanism because they adjustment of the gap between the brake shoes and the rotor disc is inherent in disc brakes. So, the disadvantage in automatic adjustment is the extra addition of hardware components to the drum. Like all hardware parts, the parts of the adjuster too get weak and face wearing. This affects the strength of braking. But however, rotor discs are commonly used only in front wheels.