Parking Brakes, Spring Brakes, and Emergency/Backup Systems
Domain 6 covers parking brakes, spring brakes, and emergency/backup systems. The exam tests how spring brakes apply, how to test the parking brake, and what happens when air pressure is lost.
Rules
- Parking brake control: yellow diamond push-pull; applies spring brakes.
- Trailer air supply: red octagon; controls trailer parking/emergency brakes.
- Spring brakes are mechanically applied by springs when air pressure is removed; air pressure releases them.
- If pressure drops to about 20–45 psi, spring brakes apply automatically.
- Parking brake test: apply it, release service brakes, try to move; the brake should hold.
- Breakaway: the emergency line separates; trailer brakes apply.
Worked example
A tractor-trailer loses air pressure to 30 psi. Reasoning: the low-pressure warning is already on, and spring brakes will apply as pressure falls into the 20–45 psi range. The vehicle should stop; it cannot be moved normally until air pressure is restored and the parking brake is released.
Traps that cost marks
Thinking spring brakes are air-applied; using the parking brake for normal stopping; confusing yellow and red knobs; not testing parking brake holding power; forcing a vehicle with spring brakes applied.