Air Brake Operation and Braking Technique
Air Brake Operation and Braking Technique
This domain tests how air pressure actually applies the brakes and how to control a heavy vehicle without overheating the brakes. Expect questions on downhill technique, brake fade, and the difference between service brakes and spring brakes.
Core rules
- Pressing the treadle valve sends air to the brake chambers. The pushrod moves the slack adjuster, which rotates the cam and presses the linings against the drum.
- Air brakes are not hydraulic. If air pressure is lost, service braking is lost, but spring brakes apply automatically as pressure falls.
- On a long or steep downgrade, select a low gear before the descent and use engine braking. Do not coast in neutral.
- Use snub braking: apply the service brakes firmly enough to reduce speed about 5-6 mph, then release and let the brakes cool. Repeat as needed. Riding the brakes lightly for a long period builds heat and causes fade.
- Brake fade means overheated linings and drums lose friction. The pedal may feel normal or hard, but stopping power drops. Stop and let brakes cool.
- After driving through deep water, test brakes at low speed and apply light pressure to dry them.
Worked example
A loaded tractor-trailer starts a 6% downgrade in a low gear. Speed still climbs from 35 to 45 mph. The driver should brake firmly to bring speed back to about 35 mph, release, and repeat, while staying in the low gear. Continuing to ride the brakes lightly would overheat them; shifting to neutral or a higher gear would make the descent worse.
Traps
Do not use the parking/spring brake as a service brake on a moving vehicle. Do not "fan" the brakes rapidly or ride them continuously downhill. Do not believe a hard pedal proves the brakes are cool and working. Do not wait until the brakes smell hot before changing technique.