5.2 Drum Brake Inspection and Shoe Replacement

5.2 Drum Brake Inspection and Shoe Replacement

Drum Brake Operating Principles

While modern passenger cars rely primarily on disc brakes, many light trucks and compact vehicles continue to use drum brakes on the rear axle. Drum brakes operate on a self-energizing design. When the brake pedal is depressed, hydraulic pressure from the master cylinder forces the two pistons of the wheel cylinder outward. These pistons push the primary (leading) and secondary (trailing) brake shoes against the inside of the rotating brake drum.

graph TD
    WC[Wheel Cylinder] -->|Pushes Outward| PS[Primary Shoe / Leading]
    WC -->|Pushes Outward| SS[Secondary Shoe / Trailing]
    PS -->|Force Transfer| SW[Star Wheel Adjuster]
    SW -->|Friction Assist| SS
    SS -->|Squeezes| BD[Brake Drum]
    RS[Return Springs] -->|Retracts| PS
    RS -->|Retracts| SS
    HS[Hold-Down Springs] -->|Clamps to Backing Plate| PS
    HS -->|Clamps to Backing Plate| SS

In a standard duo-servo drum brake design, the friction between the primary shoe and the drum forces the primary shoe downward, transferring force through an adjuster star wheel link to press the secondary shoe more firmly against the drum. This self-energizing action creates high braking force with low hydraulic pressure. However, drum brakes are less effective at heat dissipation compared to disc brakes, making them more prone to brake fade under heavy load.

Backing Plate and Shoe Inspection

The backing plate is the rigid steel foundation of the drum brake assembly, bolted directly to the axle flange. It holds the shoes, springs, wheel cylinder, and parking brake linkage.

• Lubrication Points: The backing plate features raised contact pads (ledges), typically three per shoe (six total), where the metal backing of the brake shoes slides as they expand and contract. These pads must be cleaned of rust and old grease with a wire brush, inspected for deep grooving (which can catch the shoe and cause binding), and lubricated with a thin layer of high-temperature brake lubricant. Lubrication prevents squeaks and ensures smooth shoe movement.
• Shoe Thickness: Brake shoes consist of a steel table and web with friction lining. The lining is either bonded or riveted to the table. Measure lining thickness at the thinnest point. The standard minimum thickness limit for bonded linings is 2.0mm (approx. 3/32"). For riveted linings, measure from the top of the rivet head to the friction surface; the limit is also 2.0mm above the rivet head to prevent the rivets from scoring the drum.

Drum Brake Hardware and Self-Adjusters

The drum brake assembly relies on complex spring hardware to control shoe positioning and retraction:

1. Return Springs: Heavy coil springs that connect the shoes to an anchor pin at the top of the backing plate. These springs pull the shoes back away from the drum when hydraulic pressure is released, retracting the wheel cylinder pistons. Weak or broken return springs cause brake drag, overheating, and premature shoe wear.
2. Hold-Down Springs: Small springs held by pins and washers/cups that clamp the brake shoes flat against the backing plate contact pads. They prevent lateral movement and rattling.
3. Self-Adjusters: Comprising a star wheel adjuster screw, adjuster lever (pawl), spring, and sometimes a cable or link. In most duo-servo designs, the self-adjuster mechanism operates when the brakes are applied while the vehicle is reversing. As the shoes expand in reverse, the adjuster cable pulls the lever up. When the brakes are released, the adjuster spring pulls the lever down, turning the star wheel one notch if there is excessive shoe-to-drum clearance, thereby keeping the shoes close to the drum.

Brake Drum and Wheel Cylinder Inspection

Drum Inspection and Measurement

A brake drum must be inspected for scoring, heat spots (hard blue spots caused by extreme heat), cracking, and out-of-roundness.

• Diameter Measurement: Use a drum micrometer to measure the drum's inside diameter in at least two perpendicular directions to check for out-of-roundness.
• Maximum Limit: The maximum allowable machining diameter (discard limit) is stamped on the outside of the drum (e.g., "Max Dia 9.090 inches"). If a drum measures at or near this limit, or would exceed it after resurfacing, it must be replaced. Running an oversized drum reduces heat dissipation, increases shoe travel, and can cause the wheel cylinder pistons to pop out.

Wheel Cylinder Leak Inspection

The wheel cylinder converts hydraulic pressure into mechanical force. It contains two pistons, two rubber cup seals, an expander spring, and two external rubber dust boots.

• Leak Detection: To inspect a wheel cylinder, peel back the rubber dust boots at each end. A thin film of moisture is normal for lubrication, but if active liquid brake fluid drips out, the internal cup seals are leaking. A leaking wheel cylinder must be replaced. Fluid leaks will contaminate the brake shoe friction linings, causing them to swell, grab, or slip, and will eventually lead to a low brake pedal and loss of system pressure.

Brake Shoe Replacement Steps

1. Deconstruction: Wash the assembly with brake cleaner. Use a brake spring tool to remove the return springs, hold-down springs, parking brake cable, and self-adjuster hardware.
2. Backing Plate Prep: Clean the backing plate ledges to bare metal. Inspect for grooves. Apply high-temperature brake grease to the contact pads.
3. Pre-adjust Star Wheel: Clean and lubricate the threads of the star wheel adjuster screw. Install it between the new shoes.
4. Reassembly: Reinstall the hold-down pins and springs, return springs, and adjuster linkage.
5. Initial Adjust: Adjust the star wheel using a brake spoon tool through the backing plate slot until the drum just slips over the shoes with a slight, uniform drag when rotated.

Measuring and Machining Brake Drums

Remove the drum and clean it before measurement. Use a drum micrometer or inside micrometer at several points to check diameter and out-of-round. Compare to the maximum diameter stamped on the drum. If the drum is within limits but scored, machine it on a brake lathe to restore a smooth friction surface, then remeasure—machining increases diameter, so a drum already near discard may need replacement instead of cutting. After machining, wipe away metal chips, lubricate backing-plate shoe contact pads with high-temperature brake grease (never on friction material), and pre-adjust the star wheel so the drum installs with light drag.