What type of pressure can legally hold the parking or emergency brakes on trucks or buses?

The correct answer is that spring pressure can legally hold the parking or emergency brakes on trucks or buses. This mechanism is essential for safety and reliability in brake systems, particularly in heavy vehicles.

Spring brakes work by using large, powerful springs to apply pressure and hold the brakes in place. When the driver applies the brakes, air pressure is used to release the tension on these springs, allowing the brakes to disengage. When the air pressure is released, such as during a vehicle shutdown, the springs automatically engage the brakes. This means that spring brakes are always set to apply if there is a loss of air pressure, providing a fail-safe mechanism to prevent the vehicle from rolling when parked.

In contrast, while air, hydraulic, and pneumatic pressures are all involved in different brake systems, they do not serve the same purpose for holding the parking or emergency brakes. Air pressure and pneumatic pressure primarily pertain to the operation of the brake system while the vehicle is in motion, and hydraulic pressure is often used in passenger vehicles but not generally in the heavy-duty vehicles that utilize spring brakes for parking and emergency applications.