Braking Systems and Emergency Procedures
Braking Systems and Emergency Procedures tests how air brakes behave on a combination and what to do when something fails. The exam expects brake lag, stopping distance, correct use of the trailer hand valve, ABS, retarders, and emergency responses.
Braking rules
Air brakes have lag: roughly 0.3 to 0.5 seconds before trailer brakes apply after the tractor pedal. Total stopping distance includes perception, reaction, brake lag, and braking distance. The trailer hand valve is for testing trailer brakes only; never use it to stop or park. Spring brakes apply automatically when air pressure drops. Engine retarders help on long downgrades but should be used with care on slippery surfaces. ABS is for steering control, not for shortening stopping distance.
Emergencies
For a front tire blowout, hold the wheel firmly, keep the vehicle straight, and slow gradually; do not brake hard. For brake failure, downshift, use engine braking, apply the parking brake gradually, and look for a runaway ramp. If a jackknife begins, release the brakes and steer to regain alignment.
Worked example
A driver descends a grade and the service brakes begin to fade. Pumping the pedal repeatedly worsens the problem by using air faster. The correct action is to downshift, use the retarder or engine brake, apply the brakes firmly in short snubs, and if stopping power is still lost, use the emergency system and a runaway ramp.
Traps
Using the trailer hand valve to stop; pumping air brakes; relying on ABS to prevent all skids; forgetting brake lag in following distance; parking with spring brakes instead of applying the parking brake.