The primary shoe in a dual servo brake system is shorter and always faces the front of the vehicle. This system, also known as a Bendix or dual servo brake, uses hydraulic pressure from the wheel cylinder to apply both the primary and secondary shoes, with the secondary shoe doing most of the stopping. In contrast, a leading/trailing drum brake has both shoes of the same length and operates with a wedging effect.
Servicing drum brakes often requires specialty tools, such as a hold-down tool and a spring retractor tool. Before disassembly, it’s crucial to take a picture of the brake assembly and check the drum’s discard thickness. If machining a drum, ensure proper setup with a silencer band to prevent chatter. Understanding the difference between primary and secondary shoes and leading/trailing systems is key to successfully working with drum brakes and passing relevant ASE exams.
Drum Brakes Inspection Service
