2.2 Drum-Integrated Parking Brake Service and Adjustment

Drum-Integrated Parking Brake Service and Adjustment

Quick Answer: Drum parking brakes use a mechanical lever and strut to push the shoes against the drum. Always adjust the drum brake shoes using the star wheel adjuster before attempting to adjust the parking brake cable tension.

On vehicles equipped with rear drum brakes, the parking brake mechanism is integrated directly into the rear brake shoe assembly. Rather than relying on hydraulic pressure from the wheel cylinder, the parking brake system uses purely mechanical force delivered by secondary parking brake cables. Understanding how mechanical linkages interact with drum brake shoes is critical for proper diagnosis, overhaul, and adjustment.

Fundamentals of Drum Parking Brake Integration

Rear drum brakes function as both service brakes (hydraulically driven by the master cylinder and wheel cylinders) and parking brakes (mechanically driven by cables). The parking brake hardware sits inside the drum brake backing plate assembly, operating alongside the primary and secondary friction shoes, return springs, hold-down hardware, and self-adjusting mechanisms.

In both duo-servo (where shoe forces assist each other through a floating bottom adjuster) and leading-trailing drum systems, the parking brake mechanism acts upon both shoes to expand them outward against the inner cylindrical surface of the brake drum, holding the vehicle securely stationary.

Mechanical Actuation Sequence: Lever and Operating Strut

The secondary parking brake cable enters through a hole in the backing plate and attaches to the lower end of the parking brake lever.

1. Lever Attachment: The upper end of the parking brake lever is pivot-pinned to the secondary (rear-facing) brake shoe web.
2. Cable Pull: When the driver applies the parking brake, the secondary cable pulls the bottom of the lever forward toward the front of the vehicle.
3. Strut Push: As the lever pivots around its upper pin on the secondary shoe, its upper arm pushes horizontally against the operating strut (also known as the parking brake link or cross strut). The operating strut spans the gap between the primary and secondary shoes just below the wheel cylinder.
4. Primary Shoe Contact: The strut transfers force directly across to the primary (front-facing) brake shoe, pushing it outward until its friction lining firmly contacts the brake drum.
5. Secondary Shoe Reaction: Once the primary shoe makes contact and cannot move further forward, the continued pull on the lower lever forces the upper pivot pin to push backward against the secondary shoe. This expands the secondary shoe outward against the rear of the drum.
6. Wedging Action: Both shoes are now firmly wedged against the rotating surface of the drum by mechanical force alone, preventing wheel rotation.

When the driver releases the parking brake, heavy shoe return springs pull both shoes away from the drum, forcing the parking brake lever back until it rests firmly against its metal stop on the secondary shoe web.

Component Identification and Inspection Parameters

During a comprehensive brake service or inspection, inspect all mechanical parking brake hardware for wear, distortion, and corrosion:

• Lever Pivot Pin and E-Clip: The pivot pin joining the parking brake lever to the secondary shoe must rotate freely. If rust causes this pivot to seize, the parking brake will fail to apply or will remain partially engaged when released, dragging the shoes against the drum and causing extreme frictional heat, shoe glazing, and drum heat checking.
• Operating Strut Notches and Anti-Rattle Spring: Inspect the slots at both ends of the operating strut for burrs or excessive wear where they contact the shoe webs. Most designs incorporate an anti-rattle wave washer or coil spring on the strut. If missing or damaged, the strut will rattle loudly over rough roads and cause erratic parking brake application.
• Cable End Swage Engagement: Ensure the swaged cylinder at the end of the secondary cable is correctly locked into the clevis fork on the lower end of the lever. A frayed or slipping cable end can dislodge, causing total loss of parking brake operation on that wheel.
• Lining Thickness: Because the mechanical linkage has finite travel, severely worn brake shoe linings increase the distance the lever must pivot to achieve contact. If linings are worn past specification (typically 1/16 inch / 1.6 mm minimum), replace the shoes before performing adjustments.

The Golden Rule of Adjustment

The single most critical rule in drum parking brake service is: Always adjust the drum brake shoe-to-drum clearance before adjusting the parking brake cable tension.

If the brake shoes are sitting too far from the drum surface due to wear or a frozen star-wheel self-adjuster, the parking brake pedal or hand lever will travel excessively before applying the brakes. An inexperienced technician might attempt to fix this low pedal complaint by tightening the parking brake cable nut at the equalizer. This is a severe procedural error.

Tightening the cables on an unadjusted drum brake physically pulls the parking brake levers off their resting stops on the secondary shoes. When the driver later applies the foot brake, the shoes are already partially expanded by the pulled cables. This disrupts shoe geometry, prevents the self-adjuster lever from contacting the star wheel teeth correctly, and causes the rear brakes to drag continuously, leading to rapid lining wear and brake fade.

Comprehensive Service and Adjustment Workflow

To perform a precise drum parking brake service, adhere to the following professional procedure:

1. Loosen Cable Tension: Fully release the hand lever or foot pedal. Back off the equalizer adjusting nut until visible slack exists in both secondary cables, ensuring the parking brake levers at the wheels sit fully against their secondary shoe stops.
2. Inspect and Clean Hardware: Remove drums, clean backing plates, inspect pivot pins, and verify the operating strut and return springs are installed correctly.
3. Adjust Drum Shoe Clearance: Reinstall the drums. Using a drum micrometer or brake shoe adjusting gauge, pre-adjust the star wheel. Then, working through the backing plate access slot with a brake adjusting spoon, rotate the star wheel to expand the shoes until a light, even drag is felt when spinning the wheel by hand. Back off the star wheel just enough (usually 2 to 3 teeth) so the drum turns freely with only slight lining contact. Proper shoe-to-drum clearance is typically 0.030 to 0.060 inches.
4. Adjust Equalizer Cable Tension: Once shoe clearance is verified on both rear wheels, tighten the equalizer adjusting nut to remove excess cable slack. Tighten the nut until the parking brake lever or pedal achieves full application within manufacturer specifications (typically 5 to 7 clicks for a hand lever, or 1.5 to 2 inches of pedal travel).
5. Verify Free Release: Release the parking brake and verify both rear wheels rotate freely without dragging, confirming the levers have returned completely to their stops.