Using Air Brakes While Driving
What this domain covers
How to stop, slow, and control a vehicle with air brakes in normal driving, and why air brakes behave differently from hydraulic brakes.
Normal stopping
Use controlled braking: press the pedal and hold it, adjusting pressure as needed. Do not pump the brakes. Each pump releases air and, on a long grade, wastes reserve capacity you may need later. Air brakes have lag - roughly two-tenths to four-tenths of a second - between pedal movement and brake application, because air must travel through the lines. Total stopping distance is perception distance plus reaction distance plus brake lag distance plus braking distance, which is why air-braked vehicles need more following room.
ABS
If your vehicle has ABS, the yellow lamp should go out after the bulb check. In a hard stop, hold firm, steady pressure and let the system cycle; do not pump. ABS keeps wheels from locking, but it does not necessarily shorten stopping distance on every surface and does not replace correct speed and following distance.
Worked example
At 55 mph (about 81 feet per second), a 0.4-second brake lag adds roughly 32 feet of travel before the brakes begin working. Add perception and reaction distance and the total easily exceeds a basketball court. That is the reasoning behind the following-distance rule: one second per 10 feet of vehicle length below 40 mph, plus one second above 40 mph - 8 seconds for a 70-foot rig at 55 mph.
Traps that cost marks
- Pumping the pedal during a normal stop.
- Thinking ABS guarantees a shorter stop.
- Ignoring an ABS lamp that stays on; the system is disabled and needs service.
- Folding brake lag into reaction time; they are separate.