Which statement is true about drum brake dual-servo systems?

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In a drum brake dual-servo system, the two shoes are designated as primary and secondary. The primary shoe is the shorter, initial-contact shoe that most directly engages the drum. Its engagement creates a servo action that helps push the longer secondary shoe into contact with the drum as braking force is applied. This arrangement increases braking torque without needing more hydraulic input, because the geometry of the dual-servo setup multiplies the effective force.

The other terms describe different concepts or configurations (leading/trailing refers to how a shoe behaves in some drum setups depending on rotation, while inner/outer or forward/rear aren’t the standard descriptors for this dual-servo arrangement). So, the statement that they use primary and secondary shoes best captures the essence of a drum brake dual-servo system.