To reduce application pressure to the steering axle brakes as compared to the drive axle brakes. Reduced pressure to the steer axle brakes occurs for most braking which occurs at low to medium pressures.
Up to a 40psi (275kPa) service brake application, the air pressure delivered to the steer axle brakes is 50%.
During emergency braking (high application pressure 60-70psi (414-482kPa) braking, full application pressure is delivered to the steering axle.
During emergency braking of more than 20psi (130kPa), it is imperative that you have your seat belt fastened, otherwise you are going to be doing a “bug impression” on the inside of the windshield.