An air pressure protector valve prevents the operation of the airhorn when the air reservoir pressure drops below blank psi.

• A. 60
• B. 80
• C. 100
• D. 40

Answer: (B)

The air pressure protector valve acts as an interlock in the vehicle’s air system, blocking the horn when reservoir pressure is too low. This protects braking performance by preserving enough stored air for stopping power and other functions, while still allowing the horn to sound when there is a safe, adequate air reserve. The 80 psi setting is chosen because it provides a reliable margin: above this pressure there’s typically enough air to both horn signaling and braking, while below it the horn is prevented from drawing more air that could compromise safety. The other values don’t fit because 60 psi would leave less margin and could threaten braking reliability, 100 psi would keep the horn active too much and waste air, and 40 psi is far too low to safeguard braking air reserves.