NC CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, usually by drawing filtered air in and pushing it into the supply reservoir through the governor-controlled cycle.

Valve mounted on or near the compressor that starts and stops compressor pumping to hold system pressure within a set range, typically cutting out near 120-125 psi and cutting in near 100 psi.

Filter-and-desiccant unit between the compressor and the supply reservoir that removes water vapor and oil carryover from compressed air, helping prevent freeze-ups and corrosion in valves and lines.

Tank that stores compressed air delivered by the compressor; on a typical CMV it is the wet tank feeding the primary and secondary circuits, and it must be drained of moisture regularly.

One of the two isolated service-brake air circuits, generally supplying the rear (or tractor) brakes; a leak or failure here must not deplete the other circuit's ability to stop the vehicle.

The second isolated service-brake air circuit, generally supplying the front (or trailer) brakes; it is protected by a check valve so a failure in the primary side does not drain it.

One-way valve that allows air to flow in a single direction only, used to keep one circuit or tank from back-feeding into another so a single leak cannot empty the whole system.

Pressure-limiting valve that opens to vent air if system pressure climbs too high, protecting tanks and components from over-pressurization when the governor or compressor malfunctions.

Manually or automatically operated valve at the low point of a reservoir that releases accumulated water and oil; it must be opened during pre-trip inspection until only clean air escapes.

Device that injects alcohol vapor into the air system to lower the freezing point of condensed moisture, reducing the chance of ice blocking valves and lines in cold weather.

Dashboard instrument showing system air pressure; the driver watches it to confirm the compressor is keeping pressure in the normal operating range and to detect leaks or loss of pressure.

Audible and visual alert that activates when system pressure drops below about 60 psi, warning the driver that spring brakes may soon apply automatically and that stopping is required.

The foot-operated treadle valve the driver presses to send air from the reservoirs to the brake chambers; releasing it exhausts the air and releases the brakes.

Another name for the foot brake application valve, designed so the driver feels resistance and can modulate air pressure to the brakes in proportion to pedal travel.

Valve mounted near the rear (or trailer) brakes that uses a small control signal from the foot valve to send a large volume of air to the brake chambers, speeding application and release.

Valve placed close to brake chambers that exhausts air locally when the brake is released, shortening release time and reducing the chance of dragging brakes.

Cylinder containing a diaphragm and pushrod; air pressure pushes the diaphragm and pushrod to apply the service brake, and the pushrod stroke must stay within adjustment limits.

Linkage between the brake chamber pushrod and the brake shoe that converts pushrod travel into braking force; incorrect adjustment causes long stroke and weak or uneven braking.

Distance the brake chamber pushrod travels when the brake is fully applied; excessive stroke means the brakes are out of adjustment and stopping distance increases.

Condition of the slack adjusters and linkage such that pushrod stroke stays within the manufacturer's limit; out-of-adjustment brakes are a common cause of failed inspections and weak stopping.

Self-adjusting slack adjuster that takes up clearance as linings wear; it still must be checked for correct stroke and function during pre-trip inspection and cannot be assumed to be in adjustment.

Parking and emergency brakes held applied by a heavy spring and released by air pressure; if air pressure is lost, the springs apply the brakes automatically.

Dash valve (often a yellow diamond knob) that releases air to let the spring brakes apply for parking, and supplies air to release them when pushed in.

Technique of applying the parking brake valve gradually, rather than fully, to use the spring brakes as a controlled emergency brake while keeping the vehicle under control.

The delay between pressing the brake pedal and the brakes actually applying, caused by the time air takes to travel through lines and valves; it lengthens stopping distance compared with hydraulic brakes.

Total distance traveled from perception to full stop, combining perception, reaction, brake lag, and braking distance; it grows with speed, load, and poor brake adjustment.

Loss of braking power from overheated linings and drums after repeated or prolonged braking, often from riding the brakes down a long grade instead of using a lower gear.

Descending technique of braking firmly to reduce speed by about 5 mph, then releasing, repeated as needed so brakes cool between applications instead of staying applied.

Using a lower gear so the engine and drivetrain slow the vehicle, reducing reliance on the service brakes and helping prevent fade on long downgrades.

Gravel or sand ramp built on a steep downgrade for a driver who has lost braking ability, designed to slow and stop the vehicle without a crash.

Pre-trip check with the engine off and brakes released, watching the gauge for pressure loss over one minute; excessive drop indicates an air leak needing repair before driving.

Pre-trip check with the brake pedal held down and the engine off, watching for pressure loss over one minute; a drop beyond the limit means the system leaks and is unsafe.

Water that condenses inside reservoirs as compressed air cools; if not drained it can freeze in winter or corrode valves, so reservoirs are drained at each pre-trip inspection.

Controlled stop after a brake failure or pressure loss; the driver downshifts, uses the parking brake gradually, and looks for an escape ramp or safe runoff while warning others.

Space kept behind the vehicle ahead, commonly at least one second per 10 feet of vehicle length plus one second at speeds above 40 mph, to allow for longer air brake stopping distance.

The two pressures at which the governor restarts and stops compressor pumping; normal cut-out is near 120-125 psi and cut-in near 100 psi, keeping pressure in the safe operating band.

Pre-trip step of pumping the brake pedal with the engine off to lower pressure and confirm the warning buzzer and light activate by about 60 psi before spring brakes apply.

Valve that closes off the tractor's air supply to the trailer if the trailer breaks away or its lines rupture, preserving tractor air for braking and helping set the trailer spring brakes.