Components of a Drum Brake Assembly

Drum brake assemblies typically include a brake drum, backing plate, wheel cylinder, two brake shoes (primary and secondary), return and hold‑down springs, an adjuster (star wheel) with an actuating lever or cable, a parking‑brake lever and strut, an anchor pin, and various guides and clips. Together, these parts convert hydraulic pressure and parking‑brake cable pull into controlled friction to slow or stop the vehicle.

Core Components and Their Functions

The following items are the essential parts found in most passenger‑vehicle drum brake assemblies, along with what each does and how they fit together.

• Brake drum: A cast-iron or steel drum that rotates with the wheel. The brake shoes press against its inner surface to create friction. Some designs integrate the wheel hub and studs; others use a slip‑over drum.
• Backing plate (anchor plate): A rigid plate bolted to the axle or knuckle that supports all brake hardware. It includes abutment pads where shoes slide, holes for hold‑down pins, and inspection/adjuster access ports (often sealed with rubber plugs).
• Wheel cylinder: A hydraulic cylinder with two opposed pistons that push the shoes outward when brake fluid pressure is applied. It includes dust boots, a fluid inlet, and a bleeder screw for servicing.
• Brake shoes (primary and secondary): Curved steel shoes lined with friction material. The primary (usually with shorter lining) faces the front; the secondary (longer lining) faces the rear. They press into the drum to produce braking torque.
• Return springs: High‑tension springs that retract the shoes away from the drum when hydraulic pressure is released, maintaining the correct rest position.
• Hold‑down springs, pins, and cups: Hardware that secures each shoe against the backing plate to control movement and noise.
• Anchor pin/upper anchor block: A fixed post or block at the top of the assembly where the shoes bear under load; it provides a reaction point for shoe forces.
• Adjuster (star‑wheel) assembly: A threaded screw mechanism—typically located at the bottom between the shoes—that sets and maintains the correct shoe‑to‑drum clearance. Left and right sides use opposite threads.
• Adjuster lever and cable/actuator: A lever (sometimes cable‑assisted) that turns the star wheel during brake application or when the parking brake is used, automatically compensating for lining wear.
• Parking‑brake lever and strut/link: A lever attached to the secondary shoe and a rigid strut that transfers cable pull, expanding the shoes mechanically for parking/backup braking.
• Shoe guide plates/abutment pads and lubricating points: Hardened contact surfaces on the backing plate where shoes slide; these require a thin film of high‑temperature brake lubricant.
• Anti‑rattle clips and minor hardware: Small springs and clips that stabilize components and minimize noise and vibration.
• Drum/hub and bearing (design‑dependent): Some assemblies use a drum integrated with the wheel hub and bearing (with a grease seal); others use a separate, slip‑over drum on a hub with a separate bearing unit.

Together, these core parts create a self‑energizing friction system that converts hydraulic and mechanical input into stopping force while keeping clearances and alignment within safe limits.

Design Variations and What They Change

Drum brakes share the same fundamentals, but layouts and hardware details vary by vehicle type and manufacturer. The points below highlight the common variations and how they affect the parts you’ll see.

• Duo‑servo vs. leading/trailing: Duo‑servo systems use an upper anchor with the shoes linked at the bottom by the adjuster; braking forces transfer from primary to secondary shoe for stronger reverse and forward braking. Leading/trailing designs use different anchoring that yields more linear response and are common on smaller cars’ rear axles.
• Self‑adjuster style: Some use a cable and lever actuated when reversing and applying the brakes; others use a pawl that advances the star wheel during normal stops. Left/right adjusters are mirror‑image and use opposite threads.
• Parking‑brake actuation: Many assemblies integrate a lever on the secondary shoe with a strut to the primary. Trucks may use different linkage geometry or heavier components.
• Drum and hub integration: Light vehicles often have slip‑over drums; others (especially older designs) combine drum, hub, and bearing, requiring seal and bearing service during removal.
• ABS tone ring location: While not a brake shoe component, some rear hubs include an ABS tone ring adjacent to the drum; service requires care to avoid damage.

These variations don’t change the fundamental component set, but they do affect part numbers, service procedures, and how the adjuster and parking‑brake hardware are arranged.

Typical Layout and Orientation

In most rear drum brakes, the wheel cylinder sits at the top between the shoes, the star‑wheel adjuster sits at the bottom, the primary shoe faces forward with a shorter lining, and the secondary shoe faces rearward with a longer lining. Return springs span across the shoes near the top and bottom, while hold‑down hardware anchors each shoe to the backing plate at mid‑height.

Common Hardware Kit Items Replaced During Service

Because springs and small clips fatigue with heat and time, technicians often replace a “hardware kit” along with shoes. The items below are typical of those kits.

• New return springs (primary and secondary) matched for side and position.
• Hold‑down springs, pins, and cups for both shoes.
• Adjuster lever spring, star‑wheel spring, and self‑adjuster cable (if equipped).
• Anti‑rattle clips and shoe-to-backing-plate guide plates.
• Rubber dust boots for the wheel cylinder and rubber access hole plugs for the backing plate.
• High‑temperature brake lubricant packets for abutment/guide points.

Refreshing these small parts helps ensure consistent shoe retraction, correct clearance adjustment, and reduced noise after the shoes and drums are serviced.

Summary

A drum brake assembly consists of the drum, backing plate, wheel cylinder, primary and secondary shoes, return and hold‑down springs, a star‑wheel adjuster with lever or cable, a parking‑brake lever and strut, an anchor pin, and various guides and clips. Depending on design, the drum may integrate the hub and bearing, and the self‑adjuster and parking‑brake linkages can differ. These components work together to provide reliable braking by converting hydraulic and mechanical input into controlled friction inside the drum.

What is not part of a drum brake assembly?

A caliper is not part of a drum brake system; it is a component of a disc brake system. 
Explanation:

• Drum brake components: Opens in new tabThe main parts of a drum brake include the brake drum, brake shoes, wheel cylinder, backing plate, and return springs.
• Caliper function: Opens in new tabA caliper is a part of a disc brake system, where it houses the pistons that push the brake pads against the rotating disc to slow down the wheel. 

For a more helpful explanation to multiple choice questions, try including the answer options in your search.

What’s the biggest problem with drum brakes?

The biggest problem with drum brakes is their tendency for brake fade, or reduced stopping power, due to their poor heat dissipation compared to disc brakes, which leads to longer stopping distances and potential loss of control, especially in heavy use or wet conditions. Other significant problems include their complex maintenance, susceptibility to corrosion, less effective performance in wet weather, and the potential for issues like wheel cylinder failure or seizing due to corrosion and trapped brake dust.
 
Brake Fade and Heat 

• Poor Heat Dissipation: Opens in new tabDrum brakes are an enclosed system, meaning they have limited air circulation, which prevents heat from escaping efficiently. 
• Reduced Stopping Power: Opens in new tabWhen brakes overheat, the friction material can glaze, reducing friction and thus stopping power. This phenomenon, called brake fade, leads to longer stopping distances. 
• Vulnerable in Wet Conditions: Opens in new tabThe enclosed design also traps water, slowing down the drying process and further impacting braking performance in wet weather. 

Maintenance and Corrosion

• Complex Maintenance: Drum brakes have many components, making them more complex and time-consuming to maintain and repair than disc brakes. 
• Corrosion: The internal components, such as hold-down springs and the self-adjuster mechanism, are prone to corrosion from road salt and moisture, leading to sticking and failure. 
• Stuck Components: Corrosion can cause springs and the self-adjuster to seize, which disrupts proper shoe adjustment and reduces braking efficiency. 

Other Issues

• Brake Dust Buildup: Opens in new tabDust can accumulate inside the drums, worsening heat issues and binding problems. 
• Uneven Braking: Opens in new tabWorn drums or improperly adjusted shoes can lead to uneven braking, vibrations, and difficulty stopping. 
• Wheel Cylinder Problems: Opens in new tabLeaks in the wheel cylinder or its failure to operate correctly can cause reduced braking efficiency and other issues. 

What is included in a drum brake kit?

Drum Brake System

• Brake Shoes – Rear.
• Brake Hose.
• Brake Line.
• Wheel Cylinder – Rear.
• Brake Drum – Rear.
• Wheel Seal – Rear.
• Parking Brake Shoe.
• Wheel Seal – Front.

What are the main components of a drum brake system?

In drum brake system; There are drum, wheel brake cylinder (brake center), pistons inside the brake cylinder, shoes, pads, return springs. In addition, the handbrake mechanism is located on the brake plate. Drum: It is a metal piece generally made of cast iron that rotates with the wheel.