Answer :
Final answer:
The parking brake in vehicles with air brake systems automatically engages when air pressure drops below 60 psi, ensuring safety in case of air pressure failure.
Explanation:
The parking brake in vehicles equipped with air brake systems is designed to engage automatically when the air pressure in the system drops below a certain level. This safety feature ensures that if there's a significant loss in air pressure, which could imply a leak or failure in the air brake system, the parking brakes will apply to prevent the vehicle from moving unintentionally. The correct answer to the question: Parking brake applies brakes when air pressure drops below is a) 60 psi. It's crucial for safety that air brake systems maintain adequate pressure to operate effectively, and the automatic application of parking brakes at 60 psi is a measure to prevent accidents in case of air pressure failure.
