ND CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, typically governed to cut out around 120-125 psi and cut in around 100 psi.

Valve on the compressor that monitors system pressure and controls when the compressor loads (pumps) or unloads (idles), setting the cut-in and cut-out pressure range.

Device that removes moisture and oil vapor from compressed air before it enters the storage tanks, usually with a desiccant cartridge and purge cycle to prevent freeze-ups and corrosion.

The first storage tank to receive air from the compressor and dryer; it collects moisture and passes air on to the primary and secondary tanks.

Storage tank that holds air for one half of a split braking system, commonly supplying the rear service brakes, so a leak in the other circuit does not empty it.

Storage tank that holds air for the other half of a split braking system, commonly supplying the front service brakes, keeping the two circuits independent.

Manual or automatic valve at the low point of an air tank used to blow off accumulated water and oil, which should be drained daily to prevent corrosion and brake failure.

Pressure-relief valve on the air system, usually set near 150 psi, that opens to release air if the governor fails and pressure climbs too high.

Treadle-operated valve the driver presses to apply the service brakes; it meters air pressure to the brake chambers in proportion to pedal travel.

Another name for the foot brake valve, describing the pedal-actuated plunger that controls how much air pressure reaches the service brake chambers.

Cylinder containing a diaphragm and pushrod that converts air pressure into mechanical force to apply the service brake when air enters.

Linkage between the brake chamber pushrod and the brake shoe that transmits force and provides adjustment for lining wear; it should move no more than about one inch by hand.

Rod extending from the brake chamber that moves outward when air pressure flexes the diaphragm, transferring force to the slack adjuster and brake mechanism.

Emergency and parking brake assembly with a powerful spring that applies the brake when air pressure is released, and is caged or released by air to allow the vehicle to move.

Dash valve, usually a yellow diamond-shaped push-pull knob, that releases air to apply the spring brakes and supplies air to release them when pushed in.

Mechanical bolt used to manually compress and hold the spring in a spring brake chamber when air pressure is unavailable, allowing the vehicle to be moved.

Valve on trailers that charges the trailer tanks and applies the trailer spring brakes when supply pressure is lost, acting as the trailer's emergency and parking brake control.

Dash-mounted lever that applies only the trailer service brakes independently of the tractor, useful for checking trailer brake operation and for some emergency stops.

Valve that seals the tractor air system when trailer pressure drops or the trailer line is disconnected, preventing loss of tractor air and applying trailer spring brakes.

Buzzer and light that must activate before pressure falls below 60 psi, warning the driver that braking and emergency systems are losing air.

Dash instrument showing system air pressure in psi; the driver watches it to confirm the compressor builds pressure and that leaks are not draining the system.

With the engine off, fully charged system and brakes released, the driver watches the gauge for one minute; pressure should not drop more than 3 psi for a single vehicle or 4 psi for a combination.

With the engine off and the service brakes fully applied and held, the driver watches the gauge for one minute; loss should not exceed 4 psi for a single vehicle or 6 psi for a combination.

Check that pressure rises from about 85 to 100 psi within 45 seconds at normal idle, showing the compressor and governor are functioning correctly.

Observation that the compressor stops building pressure at the governor cut-out, typically around 120-125 psi, confirming the governor unloads the compressor properly.

Test in which the parking brake valve is pulled and the vehicle is gently pulled against the brakes to confirm the spring brakes hold, then the valve is released.

Inspection of how far each pushrod extends when the brakes are applied, compared with the manufacturer's limit, to detect excessive lining or adjustment wear.

Distance the brake chamber pushrod moves when the brakes are applied; excessive travel means the brakes are out of adjustment and stopping distance increases.

Loss of braking power caused by overheating of drums and linings from repeated or prolonged use, often from riding the brakes on long downgrades.

Technique of applying the brakes firmly to reduce speed about 5-6 mph, then releasing, repeated on a downgrade to control speed while letting brakes cool between applications.

Device that uses engine compression or exhaust restriction to slow the vehicle without using the service brakes, helping prevent fade on long or steep downgrades.

Emergency technique of pressing the brake pedal fully and holding it while steering, used when brakes are fading or a quick stop is needed, rather than pumping the pedal.

Applying and holding the brakes just short of wheel lockup to get maximum stopping force while keeping steering control, especially on slippery surfaces.

Controlled stop made when a hazard appears, using firm steady brake pressure, downshifting, and steering; the driver should not pump air brakes because each application uses air.

If brakes fail, the driver downshifts, uses the engine brake, applies the parking brake gradually, and looks for an escape route such as an uphill grade or runaway ramp.

Gravel or sand escape ramp built on steep downgrades to stop a vehicle that has lost braking ability, often with a rising grade that slows and holds the vehicle.

Device that injects alcohol into the air system to lower the freezing point of moisture, helping prevent ice from blocking valves and lines in cold weather.