Fan off air pressure in tanks by stepping on and off the brake pedal to reduce tank pressure. The tractor protection valve and parking brake valve should be closed (pop out) on a tractor-trailer combination vehicle, and a parking brake valve should close on other combination and single vehicle types when the air pressure falls to the manufacturer's specification of 20-40 psi. This technique is performed to ensure:

a. Water is removed from the air tanks
b. Spring brakes come on automatically
c. Service brakes are operational
d. Leakage rate is within manufacturer's specifications.

The procedure of releasing air pressure from tanks to ensure that spring brakes automatically engage is a crucial safety measure in vehicles with air brake systems.

Explanation:

The technique of reducing air pressure in tanks by stepping on and off the brake pedal to ensure the spring brakes come on automatically is a safety procedure in vehicles with air brake systems. When the air pressure falls to the manufacturer's specification of 20-40 psi, the tractor protection valve and parking brake valve (in tractor-trailer combination vehicles) should close, as should the parking brake valve in other combination vehicles. This is critical because it ensures that the spring brakes, which are designed to automatically activate in the event of low air pressure, engage to prevent vehicle movement, particularly when the vehicle is parked or during a loss of air pressure while driving.