KY CDL Air Brakes key terms

Engine-driven pump that draws in outside air and compresses it to build and maintain the air pressure stored in the vehicle's reservoirs. It cycles on and off as pressure drops, governed by the governor.

Valve that monitors reservoir pressure and signals the compressor to load (pump) when pressure falls to the cut-in level and to unload (stop pumping) at the cut-out level, typically around 100 to 125 psi.

The upper pressure setting at which the governor stops the compressor from pumping, usually about 125 psi. Pressure above this point is normally relieved by the safety valve.

The lower pressure setting at which the governor restarts the compressor to rebuild pressure, commonly around 100 psi. A gauge that never drops to this point may indicate a faulty gauge or governor.

Storage tank that holds compressed air for braking and other systems. Most vehicles have at least two, and each tank is drained of moisture and oil during the pre-trip inspection.

Petcock or valve at the bottom of each reservoir used to blow out accumulated water and oil. Draining tanks daily prevents moisture from reaching valves and freezing or corroding them.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, helping prevent ice from blocking valves in cold weather. It supplements, but does not replace, regular tank draining.

Cartridge or desiccant unit that removes moisture from compressed air before it reaches the reservoirs. It reduces water buildup and freeze-ups; the cartridge requires periodic replacement per the manufacturer.

Pressure-relief valve on the air system that opens automatically if reservoir pressure climbs above a safe limit, protecting tanks and lines from over-pressurization. It is usually set near 150 psi.

Treadle valve the driver presses to apply the brakes. Pressing it admits air to the brake chambers; releasing it exhausts that air and lets the brakes release.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the foundation brake. A spring brake chamber also contains a powerful spring that applies the brake if air pressure is lost.

Linkage between the brake chamber pushrod and the brake shoe that converts pushrod stroke into shoe movement. It is adjusted so the pushrod stroke stays within the manufacturer's limit.

Distance the brake chamber pushrod travels when the brake is fully applied. Excessive stroke means the brakes are out of adjustment and stopping power is reduced; it is checked during inspection.

Drum brake in which an S-shaped cam rotates to spread the brake shoes against the drum. Cam rotation comes from the pushrod, so adjustment and lubrication of the cam and slack adjuster matter.

Rotating metal drum attached to the wheel that the brake shoes press against to slow the vehicle. Cracks, deep scoring, or heat discoloration on a drum are inspection defects.

Curved metal part carrying friction lining that presses against the brake drum or disc to create stopping friction. Worn or contaminated lining reduces braking and must be repaired.

Tank that stores air for the system and feeds the primary and secondary circuits through check valves. It is the first tank downstream of the compressor and dryer.

One of two separate air circuits in a dual air brake system, usually supplying the rear brakes. If it fails, the secondary circuit still provides braking, though stopping distance increases.

The second independent air circuit in a dual system, usually supplying the front brakes. Its isolation from the primary circuit lets the vehicle stop if either circuit loses pressure.

Brake system with two separate air circuits and reservoirs so a leak or failure in one circuit does not disable all braking. The driver still has braking, but performance and stopping distance worsen.

One-way valve that allows air to flow in only one direction, keeping pressure in a reservoir or circuit from bleeding back. It is what keeps the two halves of a dual system independent.

Emergency and parking brakes applied by heavy springs when air pressure is released from the spring brake chambers. They apply automatically on low pressure and are released by building air pressure.

Dash control that releases air from the spring brake chambers to apply the parking brakes and admits air to release them. It is pushed in to release and pulled out to set the brakes.

Valve that seals off the tractor's air supply and brake circuits from the trailer if the trailer breaks away or a line separates, preserving tractor braking and setting the trailer spring brakes.

Hoses, commonly color-coded supply (emergency) and service lines, that carry air between tractor and trailer. They must be coupled correctly and checked for leaks and chafing.

Coupling fittings that connect the tractor's air hoses to the trailer's. They must be locked together and checked for leaks; crossed or leaking connections cause brake problems.

Dash light and buzzer that activate when system pressure drops to an unsafe level, typically around 60 psi or the manufacturer's setting. It warns the driver before spring brakes apply automatically.

Dash instrument showing pressure in the brake circuits. In a dual system, separate needles or gauges show primary and secondary pressure so the driver can spot a failing circuit.

Gauge showing how much air pressure the driver is applying to the brakes. It helps the driver judge braking effort and notice changes in brake performance.

Test in which the brakes are applied and held while the driver watches the gauges for pressure loss. Excessive drop indicates a leak that must be found and repaired before driving.

Inspection step where air pressure is built to governor cut-out, the engine is shut off, and the driver times how long pressure holds with brakes released and then applied, comparing results to the allowed loss.

Systematic check before driving that includes draining tanks, checking compressor buildup, testing warning devices, checking spring brake application, and confirming the brakes hold the vehicle.

Check that the compressor raises pressure from a low point to governor cut-out within a reasonable time, showing the compressor and governor are working. Slow buildup signals a problem to repair.

Test in which the driver applies the brakes, holds the pedal, and confirms the vehicle does not move or creep, verifying the brakes actually hold under pressure.

Loss of braking power from overheated drums, shoes, or discs, often after repeated or prolonged braking. It is prevented by braking early and using engine braking on downgrades.

Using gearing and engine compression to slow the vehicle, especially on long downgrades, so the service brakes are not overheated. It is the primary speed-control tool on steep grades.

Technique of applying the brakes firmly to reduce speed by a set amount, then releasing them to cool, repeated down a grade. It avoids continuous light braking that overheats the brakes.

Braking used when service brakes fail or pressure is lost, such as spring brakes applying automatically or controlled use of the parking brake valve. The driver should slow gradually and find a safe stop.

Escape ramp on steep downgrades filled with gravel or sand, or an arrestor bed, that stops a vehicle whose brakes have failed. Using it is preferable to an uncontrolled crash.

Braking method in which the driver applies the brakes fully, releases them when the wheels roll again, and repeats, used to regain control on slippery surfaces. It differs from snub braking on grades.

Total distance traveled from perception to a full stop, including perception, reaction, and braking distance. Loaded vehicles and wet or icy roads increase it, so following distance must grow.