Colorado CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, usually governed to stop pumping near 120-125 psi and restart around 100 psi.

Valve on the compressor that cuts it out at the system's maximum pressure and cuts it back in at a lower pressure, controlling the compressor's load and unload cycles.

Canister that removes moisture and oil vapor from compressed air before it reaches the reservoirs, helping prevent corrosion and freeze-ups in the brake system.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, used as an alternative or supplement to an air dryer in cold weather.

Storage tank that holds compressed air for braking; supply and primary/secondary reservoirs feed the brake circuits so pressure is available even when the compressor is not pumping.

Cock or petcock at the bottom of a reservoir used to blow off accumulated water and oil; tanks should be drained daily to keep moisture out of the system.

Pressure-relief valve set slightly above the governor cut-out point that vents air to protect the reservoir and system from over-pressurization if the governor fails.

The brake pedal assembly that meters air pressure from the reservoirs to the brake chambers in proportion to how far the driver pushes the treadle.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the foundation brake; its stroke length is a key inspection and adjustment item.

Rod moved by the brake chamber diaphragm that transfers force to the slack adjuster; excessive travel of this rod signals brakes out of adjustment.

Lever on the brake camshaft that converts pushrod force into camshaft rotation and provides the means of adjusting brake clearance.

Slack adjuster that self-adjusts during normal brake applications; it still requires inspection, and a pushrod stroke beyond limits means it is not working properly.

Slack adjuster that must be adjusted by hand, typically by turning the adjusting nut or worm gear, to keep pushrod travel within the legal stroke limit.

Distance the pushrod extends when brakes are fully applied; it must stay within the limit for the chamber type, and too much stroke means weak or no braking.

Test where the brakes are applied fully and pushrod travel is measured against the chamber's limit; the standard way to verify adjustment before a trip.

Maximum allowable pushrod travel for a given chamber size and type, beyond which the brakes are considered out of adjustment and the vehicle must not be operated.

Condition where pushrod stroke exceeds the limit or adjustment is otherwise defective; the vehicle is placed out of service until the brakes are repaired and rechecked.

Audible and visual warning that activates before air pressure drops below a safe level, typically around 60 psi, alerting the driver to stop and find the leak.

Dash instrument showing system air pressure; the driver watches it to confirm the compressor is building pressure and that leakage is not draining the system.

Parking and emergency brakes held applied by heavy springs and released by air pressure; they apply automatically when air pressure falls too low.

Dash valve, usually a yellow diamond-shaped knob, that releases air to apply the spring brakes for parking; pulling it out applies them and pushing it in releases them.

Valve that closes automatically if trailer air pressure is lost or the trailer separates, keeping the tractor's air supply from escaping through a broken line.

Couplers connecting tractor and trailer air lines, color-coded for service and emergency circuits; they must be locked and sealed to prevent leaks and separation.

Check with the engine off, brakes released and then applied, that air pressure does not drop faster than the allowed rate, showing the system is leak-free.

Check that the compressor raises pressure from a low reading to governor cut-out within a reasonable time, confirming the compressor and governor are working.

How fast pressure drops during a leak test, measured in psi per minute; exceeding the allowed rate means the vehicle should not be driven until repaired.

Measures such as an air dryer or alcohol evaporator that keep moisture in the air system from freezing and blocking valves or lines in cold weather.

Controlled stop made when air pressure is lost or brakes fail, using downshifting, engine braking, and the spring brakes rather than a hard application.

Gravel or sand-filled runaway truck ramp used to stop a vehicle that has lost braking ability on a downgrade; steering into it is preferable to a runaway.

Loss of braking power from overheated drums and linings after repeated or prolonged use, common on long downgrades; gearing down helps prevent it.

Using a lower gear so the engine and driveline slow the vehicle, reducing reliance on the brakes and helping control speed on downgrades.

Safety defect found in inspection, such as excessive pushrod stroke or a serious air leak, that prohibits operating the vehicle until it is corrected.

Walk-around and in-cab check of air pressure build-up, leakage, warning devices, brake adjustment, and lines before driving, required for safe operation.

Electronic system that modulates brake pressure to prevent wheel lockup during hard braking, helping the driver keep steering control on slippery surfaces.

Valve that speeds application and release of the brakes by sending reservoir air directly to the brake chambers when signaled by the foot valve.