Final answer:
In brake units, the return springs hold the shoes against the anchor pin or support plate. Hydraulic pressure enacts the braking but does not maintain the position of the shoes when the brake is released.
Step-by-step explanation:
In both leading/trailing and duo-servo brake units, the shoes are held against the anchor pin/ support plate with return springs. The function of these return springs is to pull the brake shoes away from the drum when the brake is released. This mechanism ensures that the shoes do not drag on the drum, which would cause unnecessary wear and reduce efficiency. Hydraulic pressure is used to push the shoes against the drum to perform the braking action when the brake pedal is engaged. Meanwhile, the hydraulic fluid transmits this pressure from the master cylinder to the wheel cylinders, but it does not hold the shoes against the anchor pin or support plate. The correct answer to the question is (b) return springs.
Hydraulic brakes operate based on Pascal's principle, where the force applied at the brake pedal is transmitted through brake fluid in the hydraulic lines, resulting in equal pressure being distributed in the wheel cylinders. This uniform pressure then actuates the brake shoes to contact the drum or disc, creating the friction needed to slow down the vehicle. However, once the pressure is released, it is the return springs that retract the shoes back to their resting position..