With the engine off and the air system fully charged, you begin fanning off air pressure with the brake pedal. At roughly what pressure should the tractor protection valve and parking brake valve pop out?

Explanation

Spring-brake control valves must pop out somewhere between 20 and 45 psi, applying the parking brakes automatically before the system loses the ability to hold the vehicle. Pressures of 60 to 80 psi or higher would mean the valve trips too early, and 10 to 15 psi would leave the vehicle without brakes at a dangerously low pressure.