What Are the Main Components of Drum Brakes

The main components of drum brakes are the brake drum, backing plate, wheel cylinder, brake shoes with friction linings, return and hold-down springs, an adjuster mechanism (often a star wheel with lever/cable), a parking-brake lever/strut and cable connection, and an anchor pin or shoe links. These parts work together to convert hydraulic pressure into friction inside a rotating drum, slowing or stopping the wheel. Below, we explain how these components fit and function within typical drum-brake systems used on many rear axles today.

How a Drum Brake Is Organized

In a drum brake, the drum rotates with the wheel while a stationary backing plate carries the internal hardware. When you press the brake pedal, hydraulic pressure moves pistons in the wheel cylinder, forcing the shoes outward so their friction linings contact the inside of the drum. Springs pull the shoes back when pressure is released, while an adjuster maintains the correct shoe-to-drum clearance. A mechanical linkage allows the parking brake to apply the shoes independently of the hydraulic system.

Core Components and Their Functions

The following list outlines the essential parts found in most leading/trailing or duo-servo drum-brake designs, along with what each component does.

• Brake drum: A cast-iron or composite ring that rotates with the wheel; its inner surface is the friction surface the shoes press against.
• Backing plate (brake plate): A rigid plate bolted to the axle or knuckle that supports all internal brake hardware and shields it from road debris.
• Wheel cylinder: A hydraulic cylinder with one or two pistons that pushes the brake shoes outward when brake fluid pressure is applied; includes seals and dust boots.
• Brake shoes (primary and secondary): Curved steel shoes lined with friction material; the primary shoe (usually with shorter lining) and secondary shoe (longer lining) are oriented to optimize braking and wear, depending on design.
• Friction linings: Heat-resistant material bonded or riveted to the shoes that generates friction against the drum.
• Return springs: High-tension coil springs that retract the shoes from the drum when pedal pressure is released.
• Hold-down springs, cups, and pins: Hardware that secures the shoes to the backing plate while allowing limited sliding motion.
• Adjuster mechanism (star wheel, screw, lever, and cable/strut): A threaded spreader that maintains correct shoe-to-drum clearance; often self-adjusts during reverse braking or parking-brake use.
• Parking-brake lever/strut and cable attachment: A mechanical linkage on one shoe that the parking-brake cable actuates to apply the shoes without hydraulic pressure.
• Anchor pin and shoe links: A fixed point and associated links that shoes bear against to control movement and, in duo-servo systems, enable self-energizing action.
• Bleeder screw: A small valve on the wheel cylinder for removing air from the hydraulic circuit during service.
• Seals and dust boots: Rubber components that keep contaminants out and brake fluid in, extending component life.
• Hub/bearing (if integrated) and ABS tone ring (if present): In some designs the drum is integral with the hub/bearing; modern vehicles may include a tone ring for wheel-speed sensing.

Together, these elements convert hydraulic input into controlled friction, ensure the shoes release cleanly, keep wear uniform, and provide a reliable mechanical parking function.

Additional Hardware and Variations

Not all drum brakes are identical; automakers choose configurations based on packaging, cost, and performance. The variations below highlight what may differ across vehicles.

• Leading/trailing vs. duo-servo: Leading/trailing uses a fixed anchor and distinct primary/secondary roles; duo-servo links the shoes so rotation helps “self-energize” braking, particularly in reverse or forward depending on layout.
• Self-adjuster orientation: Some systems self-adjust during reverse braking; others adjust via the parking-brake application, with different lever and cable arrangements.
• Drum material and design: Drums can be plain cast iron, finned for cooling, or integrated with the hub; some rear disc systems use a “drum-in-hat” parking brake with miniature shoes inside the rotor hat.
• Wheel-cylinder layouts: Single- or twin-piston types, with various bore sizes to tune braking force and pedal feel.
• Noise and anti-rattle hardware: Shims, clips, and lubricated contact pads reduce squeal and vibration.

These differences affect service procedures and performance nuances but do not change the core concept of shoes expanding into a rotating drum to generate stopping force.

Maintenance Touchpoints

Routine inspection of drum brakes focuses on wear, leaks, and proper adjustment. The following checkpoints help ensure consistent performance and safety.

• Drum condition: Check inside diameter, roundness, and surface finish; replace or machine within spec.
• Shoe lining thickness and glazing: Replace when below minimum thickness or if linings are contaminated or glazed.
• Wheel cylinder integrity: Inspect for fluid leaks, damaged boots, or seized pistons; bleed air as needed.
• Springs and hardware: Replace weakened or corroded return and hold-down springs; use hardware kits during shoe replacement.
• Adjuster operation: Ensure the star wheel turns freely and the lever/cable moves correctly; set initial adjustment to slight shoe drag.
• Parking-brake cable: Check for smooth movement and full release; adjust cable tension to spec.
• Lubrication points: Lightly lube shoe contact pads on the backing plate with high-temp brake grease—avoid getting grease on linings or drum.
• Cleanliness: Remove brake dust with appropriate methods (avoid dry brushing); verify all fasteners are torqued to spec.

Addressing these items during service preserves stopping power, reduces noise, and extends component life.

Summary

Drum brakes center on a rotating drum and a stationary assembly of shoes, springs, a wheel cylinder, and an adjuster mounted to a backing plate, plus a mechanical parking-brake linkage. While designs vary, the essential components and their roles are consistent: apply friction reliably, retract cleanly, maintain proper clearance, and provide a robust parking function.