Alabama CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system reservoirs. It cycles on and off as pressure drops and rises, and it is cooled and lubricated by the engine's systems.

Valve that monitors system pressure and controls whether the compressor pumps air or idles. It cuts the compressor in at a low pressure and out at a high pressure, setting the system's operating range.

The upper pressure at which the governor stops the compressor from building more air, typically around 120 to 125 psi. Above this point the system is fully charged and the compressor unloads.

The lower pressure at which the governor restarts the compressor to rebuild air supply, commonly around 100 psi. Pressure falling to this point during operation signals normal compressor cycling.

Storage tank that holds compressed air for braking and auxiliary use. Tanks collect moisture and oil, so they are fitted with drain valves that must be opened regularly to remove contamination.

Manual or automatic valve at the bottom of a reservoir that lets water and oil sludge escape. Draining tanks daily prevents corrosion and keeps contaminants out of brake valves.

Device between the compressor and the reservoirs that removes moisture from compressed air. It reduces water buildup in tanks and lines, protecting valves and reducing the risk of freeze-ups in cold weather.

Optional device that injects alcohol vapor into the air system to lower the freezing point of moisture. It helps prevent ice from blocking valves and lines in freezing temperatures.

Pressure-relief valve on a reservoir that opens if pressure rises above a safe limit, usually around 150 psi. It protects tanks and components from over-pressurization if the governor fails.

The primary braking system used during normal driving to slow and stop the vehicle. In air brakes, pressing the foot valve sends air pressure to the brake chambers at the wheels.

System that holds a stopped vehicle in place, commonly applied by spring pressure when air is released from spring brake chambers. It can be applied by a dash control and releases when air pressure returns.

A backup braking function that applies automatically when system air pressure drops too low, or that the driver can apply manually with the parking brake control. It is separate from normal service braking.

Driver-operated valve that meters air pressure to the service brakes according to how far the pedal is pushed. Releasing the pedal exhausts air and releases the brakes.

Cylinder at each wheel where air pressure pushes a diaphragm and pushrod to apply the brakes. When air is released, a return spring retracts the pushrod and releases the brake.

Chamber design that uses a strong spring to apply the brakes when air pressure is removed, functioning as parking and emergency brake. Air pressure must be present to hold the spring compressed and the brake released.

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

Distance the brake chamber pushrod moves when the brakes are fully applied. Too much stroke means the brakes are out of adjustment and stopping power is reduced.

Setting the slack adjusters so pushrod travel stays within the allowed limit. Proper adjustment keeps brakes responsive and is checked during the walk-around inspection.

Dash warning light and buzzer that activate when system pressure falls below a set level, usually about 60 psi. It alerts the driver to stop and find the leak before braking ability is lost.

Dash instrument showing pressure in the brake system reservoirs. The driver watches it while driving to confirm the compressor is keeping pressure in the normal range.

System with two separate air circuits, often split front and rear, so a failure in one circuit still leaves some braking ability. Each circuit has its own reservoir and protection valve.

One of the two independent air circuits in a dual system, typically supplying the rear service brakes. It has its own reservoir so a leak elsewhere does not drain it.

The other independent air circuit in a dual system, typically supplying the front service brakes. It remains charged if the primary circuit fails, preserving partial braking.

One-way valve that allows air to flow in only one direction. It keeps air from flowing backward out of a reservoir or between circuits when pressure differences occur.

Valve mounted near the rear brakes that uses a small control signal from the foot valve to send a large volume of air to the brake chambers. It speeds rear brake response and shortens air line runs.

Valve that exhausts air quickly from brake chambers when the brake pedal is released, speeding brake release. It is often used on front axles to reduce lag.

Valve that closes the tractor's air lines when a trailer breaks away or a line fails, keeping tractor air from escaping. It also allows the trailer emergency brakes to apply.

Air line between tractor and trailer that charges the trailer system and signals loss of connection. If it separates, the tractor protection valve closes and the trailer brakes apply automatically.

Situation where a trailer separates from the tractor and its air lines part. The tractor protection valve shuts off air loss and the trailer's spring brakes apply, stopping the trailer.

Required check of the vehicle before driving, including air pressure buildup, leaks, warning devices, and brake adjustment. It confirms the air brake system is safe to operate before the trip begins.

Check for air leaks with the brakes released and system fully charged, watching the gauge for pressure drop over time. Excessive drop means a leak must be found and repaired.

Check for leaks with the brake pedal held down and system charged, watching for pressure loss. It reveals leaks in the service brake side that a released-brake test would miss.

Check that the compressor raises pressure from a low reading to the governor cut-out within a reasonable time at governed engine speed. Slow buildup points to compressor or governor problems.

Check that the warning light and buzzer come on when pressure drops below the set threshold, usually about 60 psi. It confirms the driver will be warned before braking is lost.

Check that the parking brake applies automatically when pressure falls to the spring brake actuation range, generally 20 to 45 psi. It verifies the emergency braking function works.

Loss of stopping power when brakes overheat from repeated or prolonged use, especially on long downgrades. Gears and engine braking should be used to reduce reliance on the service brakes.

Retarding device that uses engine compression to slow the vehicle without the wheel brakes. It helps control speed on grades and reduces heat buildup in the brake system.

Technique of braking firmly to reduce speed, then releasing the brakes to let them cool while the engine brake holds speed. Repeated snubs on a downgrade prevent overheating.

Condition where air pressure is trapped in a spring brake chamber and combines with spring force, making the brake very hard to release. It is avoided by fully releasing the parking brake before driving.

Mechanically compressing and locking a spring brake with a bolt so the brake releases when air is unavailable. It is used only for moving a disabled vehicle or for service, not for normal driving.

Emergency stopping method of pressing the brake pedal fully and holding it, without pumping, while keeping the wheels from locking. It is used when a quick stop is needed with air brakes.