Final answer:
The parking or emergency brakes on trucks or buses are legally held on by spring pressure, as springs provide a reliable mechanical means that does not rely on hydraulic pressure.
Step-by-step explanation:
The parking or emergency brakes on trucks or buses can be legally held on by spring pressure. The application of Pascal's principle in hydraulic systems, used for standard braking operations, suggests that when a driver exerts a force on the brake pedal, the pressure is increased first by a simple mechanical lever and then by the hydraulic system. The hydraulic system uses fluid to transfer force and equalize pressure across the braking system. However, for parking or emergency brakes, it's crucial to have a mechanical means that does not rely on hydraulic pressure that could potentially fail; thus spring-operated brakes are used to ensure that the brakes can be engaged even when the vehicle's engine is not running or if there is a loss of hydraulic pressure.