Parts of a Drum Brake Assembly: A Complete Breakdown

A drum brake assembly typically includes the brake drum, backing plate, wheel cylinder, brake shoes (primary and secondary) with friction linings, return and hold-down springs, a self-adjuster (star wheel, lever, cable), a parking-brake lever/strut, and supporting hardware such as the anchor pin and clips. These components work together to convert hydraulic pressure into friction at the wheel, slow rotation, and automatically maintain proper shoe-to-drum clearance as the linings wear.

The core friction and actuation components

These are the parts that directly create and control braking force by pressing friction material against the inside of the drum when you apply the brake pedal.

• Brake drum: The cast-iron (or composite) cylinder that rotates with the wheel; its inner surface is the friction track for the shoes.
• Backing plate: The rigid mounting plate that supports all brake hardware and serves as a partial dust shield; has lubricated pads (shoe bosses) where shoes slide.
• Wheel cylinder: A hydraulic cylinder at the top of the assembly with pistons on each end that push the shoes outward; includes dust boots and a bleeder screw.
• Brake shoes: Curved steel shoes with bonded or riveted friction linings; typically a primary (shorter lining, toward the front) and a secondary (longer lining, toward the rear).
• Anchor pin (or anchor block): The fixed post near the top where shoes and springs locate; resists braking forces as shoes expand.

Together, these pieces transform pedal pressure into outward shoe movement, contact the drum, and provide the structural foundation that keeps the assembly aligned and effective.

Return and shoe-retention hardware

These springs and fasteners retract the shoes when you release the pedal and hold the shoes in position against the backing plate under all conditions.

• Return springs: Typically two color-coded upper springs that pull shoes back from the drum after braking.
• Hold-down springs, pins, and cups: Vertical pins with springs and caps that keep each shoe pressed against the backing plate to prevent chatter.
• Anti-rattle springs/clips: Small clips or springs that reduce noise and vibration in the hardware.
• Shoe guide plates or retainers: Metal guides that help position the shoes and prevent lateral movement.

Properly functioning return and hold-down hardware ensures consistent shoe retraction, reduces noise, and prevents accelerated wear on both linings and drum.

Self-adjusting mechanism

As the linings wear, the self-adjuster keeps the shoe-to-drum clearance within spec, maintaining a firm pedal and even braking performance.

• Star wheel adjuster (screw assembly): A threaded spreader at the bottom between the shoes; rotation lengthens or shortens shoe spacing.
• Adjuster lever (pawl): A lever that engages the star wheel teeth to advance adjustment during certain brake applications.
• Adjuster cable and guide (or link): Transfers shoe movement to the pawl; includes a small pulley/guide on many designs.
• Adjuster return spring: Ensures the pawl disengages from the star wheel when not adjusting.
• Inspection/adjuster hole plug: A rubber plug in the backing plate for manual inspection or adjustment.

Most systems adjust when braking in reverse or during hard rear-brake applications; star wheels often use opposite threads side-to-side (left-hand thread on the left assembly), so correct orientation is crucial.

Parking brake linkage

The parking/emergency brake uses a mechanical pathway to expand the shoes without hydraulic pressure, holding the vehicle when parked or in a hydraulic failure.

• Parking brake lever: A lever riveted to one shoe (typically the secondary) that the cable pulls.
• Parking brake strut (or spreader bar): Transfers force across to the opposite shoe to apply both shoes evenly.
• Parking brake return spring: Helps release the lever and strut after the parking brake is disengaged.
• Cable end and equalizer (external interface): Connects the in-cabin hand lever or foot pedal to the drum brake lever; equalizes force to both rear wheels.

A smooth, free-moving parking brake linkage is essential for reliable holding force and to prevent drag that can overheat the brakes and wear linings prematurely.

Supporting parts and interfaces

These components support the drum brake assembly, seal and lubricate the wheel end, or interface with the vehicle’s broader braking and chassis systems.

• Wheel hub and bearing (often integrated with the drum): Supports vehicle weight and allows the drum to rotate.
• Grease or oil seal: Keeps lubricant in and contaminants out; critical for drum-and-axle setups.
• Wheel studs and axle flange: Provide the mechanical connection for the wheel and drum to the axle or hub.
• Bleeder screw (on the wheel cylinder): Allows air removal during brake bleeding.
• Lubrication points on backing plate pads: Small raised pads where shoes slide; require high-temperature brake grease.
• Rubber boots and grommets: Seal the wheel cylinder pistons and guide the self-adjuster cable.
• ABS tone ring/sensor (if integrated at the hub): Monitors wheel speed; not a braking force component but packaged near the drum on many vehicles.

While not all of these are strictly “friction” parts, their condition directly affects brake longevity, pedal feel, and overall safety.

How the components work together

In operation, drum brakes use hydraulic force and self-energizing action to deliver strong, stable braking, while springs and adjusters keep the system responsive and quiet over time.

1. Pedal pressure builds hydraulic force in the rear circuit; the wheel cylinder pistons push both shoes outward.
2. The primary shoe (front) typically engages first; rotation can “wrap” the shoe into the drum, amplifying force (self-energizing).
3. The secondary shoe (rear) supplies additional braking, especially in duo-servo designs, where force transfers through the adjuster to the anchor.
4. When you release the pedal, return springs retract the shoes; hold-down springs keep them aligned on the backing plate pads.
5. As linings wear, the adjuster lever advances the star wheel during reverse or hard stops, maintaining correct clearance and a consistent pedal.
6. Pulling the parking brake cable rotates the shoe-mounted lever, spreading the shoes via the strut to hold the vehicle stationary.

This interplay of hydraulic pressure, mechanical leverage, and spring tension delivers consistent braking, automatic wear compensation, and reliable parking hold.

Variations by design

Drum brake assemblies differ slightly by vehicle class and manufacturer, especially in how they balance self-energizing force and pedal feel.

• Duo-servo: Common on trucks and larger vehicles; both shoes can self-energize, using a floating adjuster at the bottom and an anchor at the top for strong reverse and forward braking.
• Leading/trailing (non-servo): Often used on small cars; one leading and one trailing shoe; more linear feel and less sensitivity to rotation direction.
• Adjuster orientation: Left vs. right assemblies may use opposite-thread adjusters; incorrect installation prevents proper self-adjustment.
• Parking brake integration: Some designs attach the lever to the secondary shoe; others vary the strut and return spring geometry.

Knowing which design you have helps ensure you install springs and adjusters in the correct orientation and diagnose performance differences accurately.

Inspection and common wear points

Regular inspection prevents reduced braking power, noise, and uneven wear, and it helps catch hydraulic leaks early.

• Shoe linings: Check thickness, glazing, oil contamination, and even contact patterns.
• Drum condition: Measure inside diameter, taper, and out-of-round; machine or replace if beyond spec.
• Wheel cylinder: Inspect for fluid leaks under dust boots; sticky pistons cause drag or pull.
• Return and hold-down springs: Replace if corroded, weakened, or distorted; weak springs cause noise and drag.
• Adjuster: Ensure free movement and correct thread direction; clean and lightly lubricate threads with high-temp brake lubricant.
• Backing plate pads: Look for grooves; lightly dress and apply proper brake grease to prevent shoe bind and squeal.
• Parking brake linkage: Verify lever pivots and strut move freely; seized parts cause poor release.

Treat brake dust carefully; use approved cleaners and avoid compressed air to limit airborne particles, especially on older vehicles that may have legacy materials.

Summary

A drum brake assembly centers on the drum, backing plate, wheel cylinder, and two shoes, supported by return and hold-down springs, a self-adjuster system, and a parking brake linkage. Correctly assembled and maintained, these parts deliver dependable braking, automatic clearance compensation, and secure parking-hold performance across a wide range of vehicles.

What are the components of the drum brake assembly?

Drum Brake Components

• Backing Plate. The backing plate is one of the drum brake components in the form of a metal-based band which is relatively thin and is placed on the back of the drum brake system.
• Wheel Cylinders.
• Brake Shoes and Pads.
• Return Spring.
• Brake Shoe Holder.
• Brake Shoe Adjuster.
• Parking Brake Lever.
• Drum Brake.

What are the three parts of the drum brake system?

Drum brake components include the backing plate, brake drum, shoe, wheel cylinder, and various springs and pins.

• Backing plate.
• Brake drum.
• Wheel cylinder.
• Brake shoe.

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 parts need to be replaced on drum brakes?

With the shoes and related system components removed, clean and inspect each item. It’s always a good idea to install new brake springs. Wheel cylinder replacement is also recommended if you see any wetness around the rubber ends, or the rubber seals are cracked.