Normal Operating Procedures and Braking Technique
Driving with air brakes
Air brakes are not instantaneous like hydraulic brakes - there is a lag while pressure travels to the chambers. Apply the pedal firmly and hold it; fanning or pumping the pedal wastes air and can leave you with less reserve when you need it. Use progressive braking: build pressure smoothly to the amount needed and hold it, releasing only when the stop is complete or speed is controlled. On long downgrades, select a low gear before the descent so engine braking and any retarder carry most of the load, and use the brakes in short, firm applications rather than riding them.
Pressure management
Watch the gauge as you drive. Normal operating pressure is roughly 100–125 psi. If pressure falls toward 60 psi, the warning will sound - get off the road and find the leak before continuing. Do not rely on the compressor to keep up with continuous brake use on a grade; if pressure keeps dropping, stop and let it build.
Traps and a rule of thumb
Test questions target three habits. First, do not fan the brakes - each application and release spends reservoir air. Second, do not ride the brakes down a grade; heat fades the linings and can glaze or crack drums. Third, do not shift to neutral to save fuel on a downgrade; you lose engine braking and retarder effectiveness. A related trap: a retarder does not replace the service brakes and does not work as a parking brake - it only slows the vehicle.
Rule of thumb: gear down before the grade, brake firmly and briefly, and keep the gauge above 100 psi whenever possible. If you must choose between a runaway and a brake fire, choose the runaway - use the escape ramp.