Drum Brakes are often found only at the rear of the vehicle. Like the disc brake, the drum brake has two brake shoes and a piston. But the drum brake also has an adjuster mechanism, an emergency brake mechanism and lots of springs. When you apply the brake pedal, hydraulic pressure from the master cylinder pushes the two pistons inside each wheel cylinder outwards to force the brake shoes against the drums and apply the brakes.. That's pretty straightforward, but why do we need all of those springs? This is where it gets a little more complicated. Many drum brakes are self-actuating. That is as the brake shoes contact the drum, there is a kind of wedging action, which has the effect of pressing the shoes into the drum with more force. The extra braking force provided by the wedging action allows drum brakes to use a smaller piston than disc brakes. But, because of the wedging action, the shoes must be pulled away from the drum when the brakes are released. This is the reason for some of the springs. Other springs help hold the brake shoes in place and return the adjuster arm after it actuates.
The two most common problems with brake drums are worn out brake shoes and leaking wheel cylinders. Because of their hidden location inside the brake drums, wheel cylinders cannot be visually inspected unless the drums are removed. Usually this leak is discovered during a service or when changing or adjusting the rear brake shoes, when drum removal reveals an unholy mess of wet brake shoe and brake dust along with more work than previously planned for. The moisture trapped in the brake fluid has literally eaten away at previously smooth walls of the wheel cylinder and brake fluid has escaped past rubber piston seal.
To prevent this situation from occurring in the first place regular brake fluid flushes are required. Corrosion won't ever take hold.
