Alberta Air Brake (Q) Knowledge Test key terms

Engine-driven pump that pulls in outside air and squeezes it into the reservoirs, supplying the pressure that operates the vehicle's brakes and other air devices.

Pressure-sensitive valve mounted on the compressor that switches the compressor between pumping and unloaded idle so system pressure stays within its designed range.

The high pressure setting, usually about 830 to 930 kPa (120 to 135 psi), at which the governor stops the compressor from adding more air to the system.

The lower pressure setting at which the governor allows the compressor to start pumping again; it is normally at least 70 kPa (10 psi) below cut-out pressure.

Canister that removes water vapour and oil from compressed air before it enters the reservoirs, reducing corrosion and preventing moisture from freezing in the lines in winter.

First tank the compressor fills; most moisture and dirt settle here before the air passes through a check valve into the primary and secondary reservoirs.

Spring-loaded relief valve on a reservoir that opens by itself if pressure climbs above a safe limit, letting air escape so the tank cannot burst.

Petcock or cable-operated valve at the bottom of each air tank; opened daily to blow out trapped water and oil that would otherwise damage the system.

Brake pedal valve operated by the driver; it meters air from the reservoirs to the brake chambers in proportion to how far the pedal is pushed.

Valve mounted close to the brake chambers that uses a small air signal from the foot valve to send a large volume of air quickly to the chambers.

Valve near the brake chambers that dumps their air to the atmosphere locally instead of sending it back through long lines, so the brakes release faster.

System with two separate circuits, primary and secondary, each with its own reservoir, so a failure in one circuit still leaves some braking ability.

Dash instrument showing pressure in the brake circuits; the driver watches it because brake performance and the warning devices depend on adequate pressure.

Coupling fittings that connect the tractor's service and supply air lines to the trailer's, letting air pass between the two units.

Round housing with a diaphragm inside that turns air pressure into a mechanical push rod movement, which applies the wheel brakes.

Rod attached to the brake chamber diaphragm; air pressure pushes it outward and its movement is transferred through the slack adjuster to the brake camshaft.

Lever on the brake camshaft that multiplies push rod force and takes up the clearance caused by lining wear, keeping brake adjustment correct.

Slack adjuster that readjusts itself during normal brake use as linings wear, but still must be inspected and must not be manually over-adjusted.

Distance the push rod moves from released to fully applied; a stroke longer than the manufacturer's limit means the brakes are out of adjustment.

The wheel-end parts that actually create stopping force: brake drum or disc, shoes or pads, cam, and slack adjuster.

Drum brake in which an S-shaped cam rotates and spreads the brake shoes outward against the drum to slow the wheel.

Heavy metal cylinder that turns with the wheel and provides the friction surface the brake shoes press against.

Friction material attached to the brake shoes; it rubs on the drum, turning motion into heat, and wears away over time.

Buzzer, light or both that come on when brake system pressure falls below about 420 kPa (60 psi), warning the driver to stop safely.

Test with the engine off and system fully charged, brakes released then applied, that times the pressure drop to show whether air is leaking.

Timed check from a low gauge reading up to full system pressure that shows the compressor raises air pressure fast enough to meet the standard.

How much pressure the system loses per minute during a leak test; too much loss means the vehicle is unsafe and must be repaired before use.

Brake chamber with a strong spring that applies the brakes whenever air pressure is removed, so it works as the parking and emergency brake.

System that holds a parked vehicle still; on air-braked vehicles it usually works by releasing air from the spring brake chambers.

When spring brake force and service brake force push on the same brake at once, which can damage the mechanism; an anti-compounding valve prevents it.

Valve that blocks service brake air from adding to the spring brake force when the parking brake is already applied, protecting the brake parts.

Valve that closes automatically when a trailer line breaks or separates, holding air in the tractor so its brakes still work.

Daily walk-around and in-cab check of compressor, tanks, lines, chambers, slack adjusters, warning devices and brake operation before the vehicle is driven.

Check where the driver applies the trailer brakes and gently pulls forward against them to confirm the trailer brakes actually hold the unit.

Loss of stopping power when brakes overheat from repeated or long use; the linings grip less and the pedal may feel different as stopping distance grows.

Downhill technique of braking firmly to cut speed by about 10 km/h, then releasing, repeated down the grade so brakes cool between applications.

Device such as an engine brake or hydraulic retarder that slows the vehicle using engine or driveline resistance, saving the service brakes for stopping.