Illinois CDL Air Brakes key terms

An engine-driven pump that draws in outside air and compresses it to build and maintain the air pressure stored in the vehicle's tanks. It runs whenever the engine runs and cycles on and off as pressure drops.

A valve on or near the compressor that senses system pressure and signals the compressor to stop pumping at a set cut-out pressure, then to resume at a lower cut-in pressure, keeping pressure within a safe operating range.

The upper pressure limit, typically around 120 to 125 psi in most heavy trucks, at which the governor stops the compressor from pumping. Pressure above this level is released by the safety valve if it keeps rising.

The lower pressure limit, usually about 100 psi, at which the governor restarts the compressor after pressure has fallen. Knowing the cut-in point helps a driver judge whether the compressor is cycling normally.

A metal storage tank that holds compressed air for the brake system. Tanks are drained of moisture and oil at the end of each day through drain cocks, and each tank supplies a specific part of the brake system.

The first tank in the system to receive air from the compressor, so named because moisture and oil tend to collect there. It is drained regularly and usually supplies air to the rest of the system through a one-way check valve.

A small valve at the bottom of an air tank that is opened to blow out accumulated water and oil. Draining daily prevents corrosion and keeps contaminants from reaching brake valves and other components.

A device between the compressor and the first tank that removes moisture from compressed air before it enters the system. It reduces water buildup in tanks and lines and helps prevent freeze-ups in cold weather.

A spring-loaded valve on the air tank that opens automatically and releases air if system pressure rises above a safe level, protecting tanks and lines from over-pressurization. It is usually set near 150 psi.

The primary braking system used during normal driving to slow and stop the vehicle. In an air brake vehicle it is operated by the foot treadle valve, which sends air pressure to the brake chambers.

A spring-applied, air-released brake that holds the vehicle when parked and also applies automatically if air pressure is lost. It is controlled by the yellow diamond-shaped push-pull knob on the dash.

The foot-operated brake valve that the driver presses to apply the service brakes. It meters air pressure to the brake chambers in proportion to how far the pedal is pushed, and releases air when the pedal is let up.

A cylinder containing a diaphragm that converts air pressure into mechanical force. Air entering the chamber pushes the diaphragm and push rod, which moves the slack adjuster and applies the brake.

The rod that extends from the brake chamber when air pressure pushes the diaphragm. Its travel distance shows how far the brake mechanism has moved and is a key indicator of brake adjustment.

The lever that connects the push rod to the brake camshaft and takes up clearance as brake linings wear. Automatic slack adjusters adjust themselves; manual ones require periodic adjustment by a qualified person.

A foundation brake in which a rotating S-shaped cam spreads brake shoes against the drum. The camshaft is turned by the slack adjuster, which is moved by the push rod when air enters the chamber.

The rotating metal drum attached to the wheel hub that the brake shoes press against to create friction and slow the wheel. Drums can crack, overheat, or become out of round.

The friction material bonded to the brake shoes that contacts the drum. Lining wears thin with use, and worn or contaminated lining reduces stopping power and can cause pulling or uneven braking.

A brake chamber design in which a heavy spring applies the brake when air pressure is released. Air pressure holds the spring compressed during normal driving, so a loss of pressure causes the brake to apply automatically.

The dash valve, usually a yellow diamond-shaped knob, used to release and apply the parking and emergency brakes. Pulling it out applies the spring brakes; pushing it in releases them once air pressure is adequate.

A brake system with two separate air circuits, each with its own tank and lines, so that a failure in one circuit still leaves the other able to stop the vehicle. One circuit typically serves the front axle and the other the rear.

The air circuit that supplies the rear service brakes in a dual system. It is usually charged first and is the circuit most drivers test first during a pre-trip inspection.

The air circuit that supplies the front service brakes in a dual system. It is separate from the primary circuit so that a loss of pressure in one circuit does not disable the other.

A valve that allows air to flow in only one direction. In a dual system, check valves keep air from flowing back from one circuit into another, preserving pressure in the good circuit after a failure.

A warning light and buzzer that activate when system pressure falls to an unsafe level, typically around 60 psi or lower. It warns the driver to stop before spring brakes apply automatically.

The dash gauge that shows the pressure in the air brake system. In a dual system there are usually two gauges or a gauge with two needles, one for each circuit, so a failure can be identified.

A check performed with the engine off and the brakes applied to see whether air pressure holds steady. With the service brakes fully applied, pressure should not drop more than a set amount in one minute.

A test in which the driver holds the brake pedal down firmly and watches the gauge for a slow loss of pressure. A steady drop indicates a leak in the service brake system that must be repaired.

A test in which the driver notes the pressure, runs the engine at normal idle, and times how long it takes the system to build from about 85 to 100 psi, confirming the compressor is working properly.

The distance the push rod moves when the brakes are applied. Excessive travel means the brakes are out of adjustment; the driver checks it by marking and measuring the rod or using the vehicle's specified limit.

The setting of the slack adjuster so that push rod travel stays within the manufacturer's limit. Out-of-adjustment brakes are the most common cause of brake-related out-of-service violations at roadside inspections.

The systematic check a driver performs before driving, covering air pressure buildup, leaks, warning devices, brake adjustment, and brake response. It is required and must be documented in the driver's record of duty status.

The loss of stopping power that occurs when brakes overheat from repeated or prolonged use, causing the friction material to lose effectiveness. Fade is managed by using a lower gear and engine braking on long downgrades.

A device that uses engine compression to slow the vehicle without using the service brakes. It helps control speed on downgrades and reduces heat buildup in the brake drums and linings.

A technique for long downgrades in which the driver applies the brakes firmly to reduce speed by about 5 mph, then releases them and lets the vehicle speed up again before repeating the cycle.

The brake system used when the service brakes fail, typically the spring brakes applied by pulling the yellow push-pull knob. It is applied gradually to avoid locking the wheels and losing control.

A controlled emergency stopping method in which the driver applies the brakes as hard as possible without locking the wheels, releases when the wheels lock, and reapplies, repeating until the vehicle stops.

The authorization added to a CDL that permits operation of vehicles equipped with air brakes. Illinois requires it for air brake vehicles, and without it a driver may not legally operate them.