In a vehicle with a dual air brake system, what holds back the parking brake during normal driving conditions?

• A. Hydraulic pressure
• B. Mechanical force
• C. Air pressure
• D. Spring tension

Answer: (C)

During normal driving conditions in a vehicle equipped with a dual air brake system, the parking brake is held in position by air pressure. In these systems, air pressure is used to engage and disengage the brakes. The parking brake is typically applied using an air pressure system that releases the brake when the driver presses the brake pedal. When the air pressure is present, it keeps the parking brake disengaged, allowing the vehicle to move freely.

This reliance on air pressure is a key aspect of air brake systems, as it provides consistent and reliable braking performance under normal driving situations. The mechanism may employ spring tension to engage the brakes when the air pressure is released (such as when the driver activates the parking brake), but during regular operations, the active component holding the parking brake in its released position is indeed the air pressure.