Final answer:
The variation in self-adjustment in leading/trailing drum brakes pertains to how the automatic adjustment mechanism compensates for wear on the brake shoes, which can lead to uneven wear and reduced braking efficiency due to disparities in adjustment rates.
Step-by-step explanation:
The variation in self-adjustment in leading/trailing drum brakes refers to the system's ability to maintain optimal gap between the brake shoes and the drum as they wear down. Leading/trailing drum brakes are so named because one of the brake shoes is positioned to be more effective during forward motion (leading) and the other during reverse (trailing). These brakes typically have an automatic adjustment mechanism that compensates for wear by adjusting the rest position of the brake shoes. However, the variation in self-adjustment is often a result of disparity in the rate at which this mechanism adjusts each brake shoe, potentially leading to uneven wear and reduced efficiency. Factors contributing to variation in self-adjustment include the condition of brake components, the quality of the adjustment mechanism, and the driving habits of the vehicle operator.