Parking Brake Rollback on a Loaded Delivery Truck
A driver parks a 2019 International DuraStar 26-foot box truck with air brakes, loaded to 25,500 lb, on a 6% uphill driveway with no curb. The asphalt is dry. The driver sets the parking brake, places the transmission in neutral, releases the service brake, and the truck rolls backward about 3 feet before stopping. The driver then sets the parking brake harder and hears a continuous hiss near the rear axle. The pressure gauge drops from 105 to 85 psi in 30 seconds. The driver needs to make a 10-minute delivery and plans to leave the truck parked there. The truck has wheel chocks in the side box but they have not been used.
1. What do the rollback and the hiss indicate, and why is leaving the truck parked there unsafe?
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A properly functioning spring parking brake should hold the vehicle without a 3-foot roll on a 6% grade. The rollback shows the parking brake is not holding effectively. The continuous hiss near the rear axle and the 20 psi drop in 30 seconds show an air leak in the parking/emergency brake system, likely at a spring brake chamber, hose, or fitting. That leak matters because the spring brakes depend on air pressure to release and on mechanical spring force to hold. A leak can cause the brake to apply unevenly or lose holding force, and the pressure drop may soon affect other air-operated components. Leaving a 25,500-lb truck on a 6% grade with a known parking brake defect and an active leak is unsafe: the truck could roll, strike someone, or damage property. The driver should not treat the delivery as more important than securing the vehicle.
2. What should the driver do before leaving the vehicle, and what should be checked?
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The driver should not leave the truck parked on the grade as it is. First, keep the service brake applied, place the transmission in the lowest reverse gear for an uphill park, and set the parking brake. Then install wheel chocks on the downhill side of the tires. If possible, move the truck to level ground before doing the delivery. If it must remain on the grade temporarily, turn the front wheels toward the side of the road so the vehicle would roll away from traffic if it moved. The driver should then check the parking brake linkage and adjustment, the spring brake chambers, the hoses and fittings near the rear axle, and the relay valve area for the source of the hiss. The vehicle should be repaired before it is returned to service. The reasoning is that chocks and a level spot reduce the immediate hazard, but they do not fix the defective parking brake or the air leak.