Drum Brake Systems: How They Work and How to Service Them Correctly

Why Drum Brakes Are Still Used

Disc brakes on the rear are more expensive to manufacture and package. They require a separate parking brake mechanism — either a secondary set of small drum brakes inside the rotor hat, a cable-actuated caliper mechanism, or an electric parking brake actuator. All of that adds cost and complexity.

Drum brakes, by contrast, are inexpensive, self-contained, and naturally integrate a parking brake function through the same shoes. On a light vehicle where rear brakes handle only 30-40% of braking effort, a properly maintained drum brake system is entirely adequate. It is not old technology — it is cost-appropriate technology for the application.

You will find rear drum brakes on base-trim economy cars, many pickup trucks, cargo vans, and light commercial vehicles. Understanding how to service them correctly is not a niche skill — it is a bread-and-butter competency for any tech working general automotive service.

Component Breakdown

The drum brake assembly consists of several key components:

Backing plate: The foundation of the assembly, bolted to the axle housing or spindle. All other components mount to it. The backing plate does not rotate with the wheel.

Wheel cylinder: The hydraulic actuator. It contains two pistons that move outward under hydraulic pressure, pushing the tops of the brake shoes against the drum. A rubber cup seal on each piston prevents fluid from bypassing the piston. The wheel cylinder has a bleeder screw for bleeding air from the system.

Brake shoes: Curved metal platforms with friction material bonded or riveted to the outer face. The shoes pivot at the bottom anchor pin and are pushed outward at the top by the wheel cylinder. In some designs, both shoes contact the drum at the same anchor; in duo-servo designs they are linked through the adjuster.

Return springs: Pull the shoes back away from the drum when hydraulic pressure is released. There are typically two — a primary return spring and a secondary return spring. These springs are under significant tension and must be handled with appropriate tools. Using the wrong tools or skipping spring replacement causes callbacks.

Hold-down hardware: Springs and retaining pins that hold each shoe flat against the backing plate while allowing it to move outward when applied. If these are missing or weak, the shoes rattle and wear unevenly.

Self-adjuster assembly: The star wheel adjuster, adjuster lever, and connecting spring that automatically maintain proper shoe-to-drum clearance as the shoes wear.

Parking brake components: A lever attached to the secondary shoe and a cable connection that applies the rear shoes mechanically, independent of hydraulic pressure.

Primary vs Secondary Shoes

In the most common drum brake design — the duo-servo or leading-trailing design — the two shoes are not identical and are not interchangeable.

The primary shoe faces forward (in the direction of forward wheel rotation) and is typically shorter with less friction material. When the vehicle moves forward and the brakes are applied, the drum's rotation drags the primary shoe in the direction of rotation, which creates a self-energizing (servo) effect. The primary shoe is wedged against the drum by the drum's rotation, increasing its effectiveness beyond what the wheel cylinder pressure alone would produce.

This servo action at the primary shoe transmits force through the adjuster to the secondary shoe, which faces rearward. The secondary shoe receives the servo force from the primary shoe plus its own wheel cylinder force, making it the heavy-duty shoe that does most of the work. The secondary shoe is longer and has more friction material than the primary.

Installing shoes on the wrong sides — or mixing up primary and secondary — results in poor braking performance, rapid uneven wear, and potentially a dangerous brake pull. Always verify shoe orientation before installation. Some manufacturers mark the shoes; if not, the longer shoe with more lining material goes to the secondary (trailing/rear) position.

Self-Adjuster Operation

As drum brake shoes wear, the friction material gets thinner and the shoes must travel further outward to contact the drum. Without adjustment, this results in excessive pedal travel and reduced braking force. The self-adjuster compensates for this wear automatically — in theory.

In the most common design, the adjuster is a threaded star wheel between the two shoes at the bottom of the assembly. The adjuster lever (also called the actuating lever) is attached to the secondary shoe. When the shoes move outward during braking — specifically during reverse braking in many designs — the adjuster lever is dragged by the shoe movement. If the shoe has traveled far enough to indicate significant wear, the lever catches a tooth on the star wheel and advances it one notch, spreading the shoes slightly outward to reduce the gap.

The key phrase is "if the shoe has traveled far enough." Self-adjusters only adjust when the shoes are worn enough to require it. If the adjuster mechanism is corroded, seized, or incorrectly assembled, it will not advance the star wheel and the brakes will go out of adjustment as the shoes wear. This is why manual adjustment is necessary after installation and why the adjuster assembly must be clean and functional.

Some vehicles use a design that adjusts during forward braking. Know which design you are working on before assuming the self-adjuster will do its job in a specific direction.

Wheel Cylinder Inspection

The wheel cylinder is a single-use hydraulic component that will eventually leak. Peel back the rubber dust boots at each end and look for brake fluid. Any fluid weeping from behind the boot is a leak — the cylinder must be rebuilt or replaced. Do not attempt to stop a weeping wheel cylinder by pushing the boot back in place. The seal is failing and will get worse.

Also inspect for corrosion on the cylinder bore. Light surface rust can sometimes be cleaned if the cylinder is being rebuilt. Pitting in the bore means replacement — a pitted bore will destroy the new cup seals quickly.

Wheel cylinder rebuilding (replacing the rubber cup seals) is acceptable practice if the bore is in good condition. On high-mileage vehicles or any vehicle where corrosion is a concern, replacement is the more reliable choice. Replacement cylinders are inexpensive and the labor to remove the old one is the same as the labor to rebuild it.

After any wheel cylinder service — rebuild or replacement — bleed the brake system at that corner. Air introduced during the repair must be removed completely.

Drum Inspection and Machining

The brake drum is a cast iron cylinder that the shoes contact on its inner surface. Inspect it for:

Scoring: Grooves cut into the drum surface by worn shoes contacting the metal shoe platform rather than the friction material. Light scoring can be machined out. Deep scoring (deeper than the amount that can be removed and still stay within maximum diameter) requires replacement.

Maximum inside diameter: Cast or stamped on the drum. Never machine the drum beyond this spec — the wall becomes too thin and the drum can crack under braking stress. Measure the drum inside diameter with a drum micrometer before and after machining.

Bell-mouth condition: When a drum wears more at the open edge than at the closed edge, it develops a tapered inside diameter — wider at the opening, narrower at the closed end. This is called bell-mouth. It prevents the shoes from contacting the full drum surface and reduces braking efficiency. Measure inside diameter at multiple points from edge to edge to detect it.

Hard spots: Localized areas of the drum that have experienced extreme heat and have developed a harder crystalline structure than the surrounding material. They appear as shiny patches with a different surface texture and often cannot be machined away — the lathe tool skips over them. Hard spots cause brake pulsation and shoe glazing. Replace the drum.

Cracks: Any crack in a brake drum is a replacement condition. There is no acceptable level of cracking in a component that must contain the force of braking without fracturing.

Manual Adjustment Procedure

After installing new shoes and hardware, the self-adjuster starts at its minimum position and the shoes will not be close enough to the drum to work properly. Manual adjustment is required before the vehicle is put back in service.

With the drum removed, expand the star wheel adjuster using a brake spoon until the shoes are slightly larger in diameter than the drum — then back off the adjuster just enough so the drum can be installed with moderate resistance. The goal is to have the shoes just barely dragging on the drum surface when the drum is spun by hand.

Once the drum is installed, apply the brakes several times with the vehicle on the ground. This seats the shoes and allows the self-adjuster to take over. Then make several moderate reverse stops — accelerate to 10-15 mph in reverse, apply brakes firmly — to exercise the self-adjuster mechanism and set final adjustment.

Check pedal height after adjustment. A properly adjusted drum brake system will have a firm pedal with normal travel. If the pedal is still low after several adjustment cycles, suspect a self-adjuster that is not functioning, an improperly installed adjuster assembly, or a wheel cylinder issue.

Spring and Hardware Service

Brake return springs are under significant tension and they weaken over time with heat cycling. Weakened return springs do not pull the shoes back far enough after brake release, causing dragging, overheating, and accelerated wear. Replace springs at every brake job — they are cheap insurance against a callback.

Use the correct brake spring tool to remove and install return springs. Pliers will slip, scratch the shoe, and risk a spring snapping back into your face. A good brake spring tool set is a must-have for drum brake work.

The hold-down hardware — pins and springs — also gets replaced at every brake service. These are typically included in a brake hardware kit specific to the vehicle application. Do not reuse old hold-down hardware. Rattling shoes from weak hold-down springs generate noise complaints and uneven contact patterns.

Before assembly, lightly lubricate the shoe contact points on the backing plate (the raised pads where the shoes slide) with a high-temperature brake lubricant — silicone-based or moly-based, not petroleum grease. This prevents the shoe from galling on the backing plate and allows it to move freely under hydraulic actuation. Keep any lubricant completely away from the shoe friction surface and the drum interior.

FAQ

How does a drum brake self-adjuster work?

The self-adjuster maintains the correct shoe-to-drum clearance as the shoes wear. It is typically a threaded star wheel adjuster connected to the secondary shoe. During reverse braking (or forward braking on some designs), the parking brake lever or adjuster actuating lever moves enough to advance the star wheel one notch, incrementally expanding the shoes outward to maintain proper clearance.

What is the difference between primary and secondary brake shoes?

In a duo-servo drum brake design, the primary shoe faces forward and is shorter with less friction material. The secondary shoe faces rearward and has more friction material. The primary shoe generates a servo action that applies additional force to the secondary shoe, which does most of the braking work. Non-servo designs use equal shoes.

How do you know when drum brakes need adjustment?

Symptoms of out-of-adjustment drum brakes include a low or spongy brake pedal, excessive pedal travel before the brakes engage, and reduced stopping power. A properly adjusted drum brake should have the shoes just clear of the drum with minimal free play — a snug drag when rotating the drum by hand is acceptable, binding is not.

Can you machine a brake drum, and what are the limits?

Yes, brake drums can be machined on a drum lathe. The maximum inside diameter is cast or stamped on the drum. If machining the drum to remove scoring would exceed this diameter, replace the drum. Also replace if the drum shows deep heat cracks, hard spots (indicated by shiny patches with different surface texture), or if it is bell-mouthed (worn larger at the open edge than at the closed edge).