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5.4 Using Air Brakes

Normal Stops

To apply the air brakes during normal stops, push the brake pedal down. Control the pressure so the vehicle comes to a smooth, safe stop. If you have a manual transmission, do not push the clutch in until the engine RPM is down close to idle. When stopped, select a starting gear.

Emergency Stops

If somebody suddenly pulls out in front of you, your natural response is to hit the brakes. This is a good response if there is enough distance to stop and you use the brakes correctly

You should brake in a way that will keep your vehicle in a straight line and allow you to turn if it becomes necessary. You can use the “controlled braking” method or the “stab braking” method.

Controlled braking:

With this method, you apply the brakes as hard as you can without locking the wheels. Keep steering wheel movements very small while doing this. If you need to make larger steering adjustments or if the wheels lock, release the brakes. Reapply the brakes as soon as you can.

Stab braking:

Use only on vehicles without anti-lock systems.

• Apply the brake all the way.
• Release the brakes when the wheels lock up.
• As soon as the wheels start rolling, put on the brakes fully again. It can take up to 1 second for the wheels to start rolling after you release the brakes. If you reapply the brakes before the wheels start rolling, the vehicle will not straighten out.

Anti-lock Brakes

New trucks and truck-trailer vehicles are equipped with anti-lock brakes. The anti-lock braking system is different than the normal air-brake system but works on the same principle.

Vehicles that have anti-lock brakes have a yellow light near the driverʼs rear side of the vehicle with the letters ABS stenciled above the light. Once the driver turns on the ignition, a yellow malfunction lamp on the instrument panel will light up, briefly indicating that the vehicle has anti-lock brakes. This lamp will remain constant if there is a malfunction in the anti-lock brake system.

For normal or emergency stopping using anti-lock brakes, the driverʼs foot remains on the brake pedal in which the anti-lock module then acts as a foot pumping the air brake system. On the air-brake system the driver must pump or use stab braking in an emergency. If the anti-lock brake system fails or malfunctions, the driver must resort to stopping the vehicle by using the normal air-brake method. If an emergency arises, the driver should use the controlled or stab braking method. The anti-lock brake system should be serviced as soon as possible.

Stopping Distance

Stopping distance was discussed in Section 2: Speed and Stopping Distance. With air brakes, there is an added delay – the time required for the brakes to work after the brake pedal is pushed. With hydraulic brakes (used on cars and light /medium trucks), the brakes work instantly. However, with air brakes, it takes a little time (one-half second or more) for the air to flow through the lines to the brakes. Thus, the total stopping distance for vehicles with air brake systems is made up of four different factors:

Perception Distance
+ Reaction Distance
+ Brake Lag Distance
+ Effective Braking Distance
------------------------------
= Total Stopping Distance

The air brake lag distance at 55 mph on dry pavement adds about 32 feet. Therefore, for an average driver traveling 55 mph under good traction and brake conditions, the total stopping distance is more than 300 feet. This is longer than a football field.