California CDL Air Brakes Test key terms

Engine-driven pump that builds and maintains air pressure in the brake system, usually governed to stop pumping near 125 psi and resume near 100 psi.

Valve that senses supply tank pressure and signals the compressor to load or unload, keeping system pressure within a safe operating range.

The upper pressure setting at which the governor unloads the compressor so it stops building air, commonly around 125 psi.

The lower pressure setting at which the governor loads the compressor again to rebuild air, commonly around 100 psi.

Steel reservoir that holds compressed air for braking; a dual system has at least one tank per half so a leak in one circuit does not drain the other.

Wet tank that receives air directly from the compressor and holds it for distribution; it collects moisture and is drained during inspection.

Manually or automatically operated valve at the bottom of each tank used to release accumulated water and oil, especially during the pre-trip inspection.

Device between compressor and tanks that removes moisture and oil from compressed air, helping prevent freeze-ups and corrosion inside the brake system.

Optional device that injects alcohol vapor into the air system to lower the freezing point of moisture and reduce winter ice blockage in valves and lines.

Pressure-relief valve on the air tank that opens automatically to release air if system pressure climbs above a safe limit, usually about 150 psi.

Dashboard instrument showing pressure in the brake circuits; the driver watches it to confirm the compressor is building air and that pressure holds when applied.

Audible and visual alert that activates when brake system pressure falls below a safe level, typically around 60 psi, warning of imminent braking loss.

Brake system with two separate air circuits, one for the front axle and one for the rear, so a failure in one still leaves some braking capability.

Valve that closes automatically to keep air in the towing vehicle's tanks if the trailer breaks away or a trailer line is severed, preserving tractor brakes.

The normal foot-operated brake system that slows and stops the vehicle during routine driving by applying air pressure to the brake chambers.

Spring-applied, air-released brake that holds a parked vehicle; it engages automatically when system air pressure drops too low for safe operation.

Brake chambers containing a powerful spring that applies the brakes when air pressure is released, providing parking and emergency braking if air is lost.

Dash valve, often a yellow diamond knob, that releases air to set the spring brakes and admits air to release them when pushed in.

Cylinder where air pressure acts on a diaphragm or piston to push the pushrod, converting air pressure into the mechanical force that applies the brakes.

Rod extending from the brake chamber that transfers force to the slack adjuster; its travel distance shows whether the brakes are properly adjusted.

Lever connecting the pushrod to the brake camshaft; it converts pushrod stroke into brake application and is checked for excessive free play during inspection.

Condition in which pushrod travel stays within the allowable limit so full braking force reaches the drums; excessive travel means the brakes need adjustment.

Heavy cylindrical housing that rotates with the wheel; brake shoes press against its inner surface to create the friction that slows the vehicle.

Friction-lined curved part that presses against the rotating brake drum when the camshaft turns, generating the friction that slows the wheel.

Loss of stopping power caused by heat buildup from overuse of the brakes, which reduces friction; controlled by gearing down and braking in short applications.

Valve mounted near the rear brakes that speeds up application and release by sending air directly from a nearby tank instead of through a long line from the pedal.

Combination valve on trailers that controls air flow to trailer brakes and applies them automatically if trailer air supply is lost or the line separates.

Valve that exhausts air quickly from a brake chamber when the pedal is released, speeding brake release and reducing lag between axle sets.

Treadle valve operated by the brake pedal that meters air pressure to the brake circuits in proportion to how far the pedal is pushed.

Pre-trip check in which the brakes are applied and held with the engine off to confirm that system air pressure does not drop faster than allowed.

Pre-trip check with the engine off and brakes released that watches the gauge for pressure loss from tanks, lines, and valves before any brake application.

Check of pushrod travel with the brakes fully applied; measuring each chamber's stroke against its limit reveals out-of-adjustment brakes before driving.

Time needed for the compressor to raise tank pressure from cut-in to cut-out; an unusually long time signals a leak or a weak compressor.

Auxiliary slowing device such as an engine brake or exhaust brake that helps control speed on downgrades without using the service brakes, reducing fade risk.

Retarder that uses engine compression and exhaust restriction to slow the vehicle; it is switched off on slippery surfaces because it can cause drive-wheel skidding.

Electronic system that senses wheel lockup and rapidly adjusts brake pressure to keep wheels rolling, helping the driver retain steering control during hard stops.

Component at each wheel that reports rotation speed to the ABS controller so it can detect an impending lockup and modulate brake pressure.

Emergency technique of pressing the brake pedal firmly, releasing it when wheels lock, and reapplying, used on vehicles without working ABS to regain steering control.