Air Brake System Parking and Special Situations
Scope
Air Brake System Parking and Special Situations tests parking on grades, chocking, cold-weather care, water contamination, and long downgrades. The exam wants mechanical backup, not reliance on air pressure alone.
Core rules
- To park, apply the parking brake, place the transmission in the lowest gear, and chock the wheels when on a grade. Turn the front wheels toward the curb or edge as space allows.
- Spring brakes are the parking brakes. They are held by springs and released by air, so they do not depend on pressure staying up.
- Do not use the trailer hand valve as a parking brake. It applies only the trailer brakes and can release or cause a jackknife.
- Drain air tanks daily. In freezing weather, moisture can freeze in valves and lines; use the air dryer and check for ice.
- After driving through deep water, test the brakes at low speed. Wet brake linings may grab or pull until they dry.
- Before a long downgrade, choose a safe gear and use engine braking. Do not wait until the brakes are hot.
Worked example
A loaded truck is parked on a 6 percent grade. The driver sets the parking brake, selects the lowest gear, and chocks the wheels. If air pressure slowly leaks off, the spring brakes remain applied, and the chocks back them up. Leaving the transmission in neutral would remove one backup and is unsafe.
Traps
Do not rely on the parking brake alone on a steep grade. Do not leave the vehicle in neutral. Do not use the trailer hand valve for parking. Do not ignore water in the tanks or brakes that pull after a water crossing.