Alberta Air Brake (Q) Knowledge Test cheat sheet

Air brakes are powered by compressed air; normal operating pressure is roughly 100-125 psi, and a low-pressure warning must sound before pressure falls below 60 psi.

Supply path: compressor to air dryer to wet tank to supply tank, then to the primary (usually rear) and secondary (usually front) circuits.

The foot (treadle) valve meters application air; relay valves mounted near the axles use reservoir air to apply the chambers and shorten application time.

Quick-release valves exhaust chamber air close to the axle, speeding brake release instead of forcing air back through the foot valve.

A double check valve passes the higher of two supply pressures, so park and emergency functions still work when one circuit loses pressure.

The tractor protection valve seals the tractor air lines if the trailer separates or is disconnected, preserving tractor air for stopping.

Spring brake chambers contain two sections: the service section applies brakes with air, the spring section applies them when air is removed.

Dash gauges show primary and secondary circuit pressure; some vehicles use one gauge with two needles.

The compressor is engine-driven and is lubricated by engine oil and cooled by engine coolant; a slipping or broken drive belt stops air production.

Governor cut-out is typically about 120-125 psi; cut-in is typically about 90-95 psi, giving a normal 20-25 psi operating differential.

When cut-out pressure is reached the governor unloads the compressor so it stops pumping; when pressure falls to cut-in it allows pumping to resume.

The air dryer removes moisture, oil and dirt before air enters the reservoirs and purges collected water during each unload cycle.

Alberta build-up check: with the engine at fast idle, pressure should rise from 50 psi to 90 psi in 3 minutes or less.

A reservoir safety valve typically opens near 150 psi and protects the tank if the governor fails and pressure keeps rising.

Continuous pumping with no unload cycle points to an air leak, a stuck unloader or a faulty governor.

A restricted compressor intake, worn compressor, or slipping belt shows up as abnormally slow pressure build-up.

The brake chamber diaphragm converts air pressure into pushrod force; the slack adjuster converts that linear push into rotation of the S-cam shaft.

Pushrod stroke must stay within the limit stamped on the chamber or the manufacturer's specification, commonly about 1-3/4 to 2 inches (45-50 mm) for a type 30 chamber.

A stroke longer than the limit means the brake is out of adjustment and braking force and stopping distance are affected.

Automatic slack adjusters re-adjust during brake applications; manual adjustment should never be used to mask a stroke that keeps going long, which signals a mechanical fault.

Manual slack adjuster setup: brakes released, turn the adjusting nut to remove free play until the lining contacts the drum, then back off to the specified clearance.

Oil, grease or coolant contamination on linings, and glazed linings, reduce friction and can cause pull or wheel lock-up.

Inspect the chamber clamp band, diaphragm, pushrod, clevis pins and mounting bracket for cracks, corrosion and looseness.

Disc brakes use an air-applied caliper with an integral automatic adjuster; check pad thickness and rotor condition instead of pushrod stroke.

Build-up test: engine at fast idle, pressure should climb from 50 psi to 90 psi within 3 minutes.

Static leakage test, brakes released, engine off and system fully charged: loss must not exceed 2 psi per minute.

Applied leakage test, brakes fully applied, engine off: loss must not exceed 3 psi per minute.

The low-pressure warning light or buzzer must operate before reservoir pressure drops below 60 psi.

Spring brakes should begin to apply automatically at roughly 20-45 psi, providing emergency braking after severe air loss.

Governor cut-out must stay within the manufacturer's limit (commonly no higher than about 125-135 psi); the reservoir safety valve is a backup at about 150 psi.

To check the warning device: charge the system, shut off the engine, reduce pressure by fanning the brakes and confirm the warning activates above 60 psi.

A system that will not hold steady pressure indicates a leak and the vehicle must not be placed in service until it is repaired.

Spring brakes are applied by heavy springs and released by air pressure; any loss of air causes them to apply automatically.

The parking brake control is a yellow diamond push-pull knob: pull out to exhaust air and apply, push in to admit air and release.

Trailer controls are typically red (emergency/supply) and blue (service); the red knob also applies the trailer spring brakes.

Compounding occurs when the parking brake is applied while the service brake is still applied, putting spring force and air force on the chamber at once and risking chamber damage.

Spring brakes act as the emergency brake: they apply progressively as pressure falls through roughly 45 psi to 20 psi.

A caging bolt mechanically compresses and holds the spring for towing or repair only; a vehicle must never be driven with brakes caged.

An inversion valve on a trailer applies the trailer spring brakes when supply pressure from the tractor is lost, even if trailer tanks still hold air.

If spring brakes will not release, the cause is low pressure or a supply line fault; the vehicle must not be moved until it is corrected.

Drain all reservoirs daily with the compressor off, starting with the wet tank, then the supply, primary and secondary tanks.

Verify the compressor builds pressure to governor cut-out and that the governor cuts back in at the specified cut-in pressure.

Perform the static leak test with brakes released (2 psi per minute maximum) and with brakes applied (3 psi per minute maximum).

Test the low-pressure warning device and confirm spring brakes apply automatically as pressure falls.

Measure pushrod stroke on each chamber and inspect linings, drums, hoses, gladhands, seals and lines for wear, cracks or leaks.

Check the air dryer for proper purge operation and confirm cold-weather moisture protection is in place.

Confirm the parking brake holds the vehicle, including on a grade, before releasing it for the day's trip.

Total stopping distance equals perception distance plus reaction distance plus brake lag distance plus braking distance.

Air brake lag is roughly 0.3-0.6 seconds while air travels through the lines; at 100 km/h one second of travel covers about 28 metres.

Select a low enough gear before starting a descent so engine braking, not the service brakes, holds the vehicle's speed.

Use snub braking on long grades: a firm application to cut speed by about 10 km/h, then release and let the brakes cool, repeating as needed.

Continuous light brake pressure overheats drums and linings, causing fade, which sharply increases stopping distance.

The parking brake must never be used to control speed on a grade; locking the drive wheels can cause a jackknife or loss of control.

If brakes fade or fail, downshift, use engine braking and any retarder, and steer for an escape ramp rather than continuing the descent.

After driving through water, apply the brakes lightly to dry the linings and restore friction.