District Of Columbia CDL Permit Test - Air Brakes

Because air pressure can eventually leak away, the emergency brakes in an air brake system must be held on by mechanical force.

Most heavy-duty vehicles use dual air brake systems, in which there are two separate braking systems operated by a single set of controls. Each system operates the brakes on different axles.

In an air brake system, the air storage tanks must be drained to remove accumulated water and compressor oil. Failing to do this can cause damage. For example, water that has built up in the system could freeze in cold weather and cause brake failure.

While driving downhill, you should hold down the brake pedal until your vehicle reaches a speed 5 mph below your safe speed, then release the brake pedal. Repeat this process.

It is important to know the maximum air loss rate that is safe for your specific vehicle. A triple combination should have a leakage rate no higher than 6 psi in a minute during an applied leakage test.

Some pre-1975 vehicles have a front brake limiting valve, which has "normal" and "slippery" settings. The idea behind these valves was to limit the air pressure available to the front brakes when driving on slippery surfaces, and thereby reduce the danger of a front-wheel skid. Studies have found that this is not actually a concern, so if your vehicle has a front brake limiting valve, leave it in the "normal" position.