Using Air Brakes While Driving
Air brakes react late
The foot valve must send air down the lines before anything happens at the wheels. At 55 mph that lag adds roughly 32 feet to stopping distance before the brakes begin to work, so you brake earlier than in a car and leave a bigger following gap.
Normal stopping
- Apply the pedal smoothly and firmly, and keep the vehicle straight.
- Use controlled braking for routine stops: hard enough to slow, not hard enough to lock the wheels.
- With ABS, brake normally and do not pump the pedal; the system modulates for you.
- Release the pedal fully when the stop is complete; holding light pressure with hot brakes can glaze linings.
- At a long stop, set the parking brake rather than holding the foot valve.
Wet brakes and adjustment
After driving through water, apply light pressure while moving to heat and dry the linings. If the vehicle pulls to one side, the brakes may be wet or out of adjustment; test them at low speed before trusting them.
Worked example
You follow a car with a two-second gap at 50 mph. The car brakes hard and, by the time your brakes apply, you have closed the gap. The cause is the lag plus the heavier vehicle. The fix is a four-second or larger gap and earlier light pressure on the pedal.
Traps
Pumping the pedal does not build pressure, coasting in neutral does not save air, and fanning the brakes is an emergency or diagnostic technique, not a normal way to stop.