BC Air Brake Knowledge Test key terms

An engine-driven pump that draws in outside air, compresses it, and pushes it into the reservoirs; it keeps replenishing the air that the brakes and other air devices use up.

A control valve on the compressor that senses reservoir pressure and switches the compressor between pumping and idling unloaded, so system pressure stays inside a safe working range.

The lower pressure setting, usually about 85 psi, at which the governor lets the compressor start pumping again after pressure has dropped from brake use or leaks.

The upper pressure setting, normally between 105 and 125 psi, at which the governor stops the compressor from pumping so pressure cannot keep climbing.

A cartridge unit that takes moisture and oil out of the compressed air before it reaches the reservoirs, reducing rust, sludge and winter ice in valves and lines.

A small container of methyl alcohol that adds alcohol vapour to the air system, lowering the freezing point of water so lines and valves are less likely to ice up.

The first tank that air from the compressor enters; it collects most of the water and dirt and then feeds the primary and secondary circuits used for braking.

A spring-loaded relief valve on the reservoir that opens by itself above about 150 psi and releases air, protecting the tank and lines from over-pressurization if the governor fails.

A small tap at the low point of each reservoir. Opening it blows out condensed water and oil, which must be done daily or at pre-trip to keep the air clean.

A dash instrument that shows reservoir pressure, usually on two dials or with two needles; it tells the driver whether there is enough air stored to brake safely.

A buzzer, light or both that must activate before reservoir pressure falls to 60 psi, telling the driver that braking power is almost gone and a stop is needed.

A sealed canister with a rubber diaphragm. Air pressure pushes the diaphragm and pushrod outward to apply the wheel brakes; when air is released, a spring returns the pushrod.

A two-part canister whose rear section holds a heavy spring compressed by air. Losing air lets the spring extend and apply the brake as a fail-safe.

The rod pushed out of a brake chamber by the diaphragm. Its travel is measured with a ruler when the brakes are applied to check adjustment.

The lever that connects the pushrod to the camshaft. It turns pushrod movement into rotation of the S-cam, and its length and adjustment set the brake stroke.

A self-adjusting slack adjuster that takes up clearance during normal brake use, but it cannot correct badly worn shoes, damaged parts or a poor foundation brake setup.

A shaft with an S-shaped end that rotates inside the brake assembly and spreads the brake shoes outward against the drum to create friction.

The curved metal shoe covered with friction lining. Air pressure forces it against the turning brake drum, and the resulting friction turns motion into heat and slows the wheel.

A valve mounted close to the rear or trailer brakes. A small air signal from the foot valve makes it deliver a large volume of air quickly, speeding application and release.

A valve fitted near a brake chamber that dumps released air close to the wheel, so the brakes let go faster than if the air had to travel back to the foot valve.

The pedal valve the driver presses. It meters reservoir air to the brake chambers in proportion to pedal travel, and exhausts that air when the pedal is let up.

A valve that seals the tractor's air supply when trailer lines separate or the dash valve is pulled, so the tractor keeps enough air to stop on its own.

The coupler on the end of a flexible air hose that joins tractor lines to trailer lines. Service couplings are blue; supply couplings are red.

The red line that carries compressed air from the tractor to charge the trailer's reservoir. If it separates, the trailer loses air and its spring brakes apply.

A trailer separating from the tractor. The air lines pull apart, trailer pressure escapes, and the trailer spring brakes lock on automatically to stop the runaway unit.

The fail-safe parking and emergency brake, applied by a strong spring when air is released and held off by air pressure. It applies by itself whenever supply pressure is lost.

A push-pull dash control, usually yellow, that exhausts air from the spring brake chambers when pulled out to park, and refills them to release when pushed in.

The red dash push-pull valve that controls air to the trailer supply line. Pulling it out cuts off the trailer and applies its spring brakes.

Applying the parking brake control while the service brake pedal is still down. Both forces act on the same mechanism at once, which can bend or break brake parts.

Pumping the brake pedal on and off repeatedly down a long hill. Each release lets the vehicle speed up and each application drains air, risking low pressure and overheating.

A downhill method where the driver brakes firmly to drop speed, then fully releases and lets the drums cool before braking again, instead of holding light continuous pressure.

Weakening of the brakes caused by heat from repeated or prolonged use. Linings and drums lose friction, so the pedal feels normal but stopping power falls.

A brake that cannot apply or release because moisture in lines, valves or chambers has turned to ice, a winter risk when air dryers or alcohol evaporators are neglected.

Brakes that stay partly on after the pedal is released, often from a sticky valve, kinked hose or out-of-adjustment slack adjuster; they overheat, fade and can seize.

A stationary check with the system fully charged and the engine off: watch the gauges with the brakes released, then fully applied, to see how fast pressure drops.

The distance a pushrod extends when the brakes are fully applied, measured and compared with the legal limit to judge whether the brakes are properly adjusted.

A pre-trip check at fast idle that times pressure rising from about 85 to 100 psi, showing whether the compressor builds air fast enough for safe operation.

A check made by bleeding air off with the engine off to confirm the warning light or buzzer comes on before pressure reaches 60 psi.

The amount of pressure a system loses per minute during leak testing. A loss above the allowed limit means the vehicle must be repaired before it is driven.