Saskatchewan Air Brake flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-out pressure

The reservoir pressure at which the governor unloads the compressor, typically about 105-125 psi; above this the compressor stops building pressure.

Governor cut-in pressure

The pressure at which the governor reloads the compressor to rebuild air, typically about 85 psi; below this the compressor resumes pumping.

Low-pressure warning threshold

A warning light and buzzer must activate before system pressure falls below 60 psi (414 kPa), alerting the driver to an unsafe air loss.

Tractor protection valve function

A valve that closes when supply pressure drops to about 20-45 psi, sealing the tractor air supply so the tractor retains braking if the trailer loses air.

Dual-circuit system purpose

Separate primary and secondary circuits so that a single failure in one circuit still leaves the other circuit able to apply service brakes.

Spring brake operation

A mechanical spring applies the brake when air pressure is released; air pressure compresses the spring to release the brake, so loss of air causes automatic application.

Static leak test limit

With full pressure, engine off, and brakes released, pressure loss must not exceed 3 psi per minute for a single unit or 4 psi for a combination.

Applied leak test limit

With full pressure, engine off, and service brake applied and held, loss must not exceed 4 psi per minute single or 6 psi combination.

Low-pressure warning test

With engine off, pump the brake pedal until the warning activates; it must come on before pressure drops below 60 psi.

Spring brake test

With pressure above 90 psi, push in the parking control to confirm release, then pull it out to confirm the spring brakes apply.

Compressor build-up test

At fast idle, pressure should rise from 85 to 100 psi in about 45 seconds; a slower rise indicates a worn compressor, slipping belt, or leak.

Pushrod stroke measurement

Measure pushrod travel with the brakes fully applied and at least 90 psi; compare the result with the adjustment limit stamped on the brake chamber.

Tractor protection test

With the trailer charged, close the trailer supply valve or disconnect the glad hand; the trailer emergency brakes must set and the tractor must hold its air.

Brake fade cause

Overheated drums and linings lose friction after prolonged or continuous braking, reducing stopping power; gearing down and intermittent firm applications help prevent it.

Downhill braking technique

Select a gear that holds speed with engine braking, apply brakes firmly enough to feel a definite retard, then release to let drums cool.

Parking brake misuse

Using the spring (parking) brake as a service brake overheats the spring brake chambers and can destroy them; it is for parking only.

Low-pressure response

If the warning sounds while driving, stop safely as soon as possible; continued driving leads to automatic spring brake application at about 20-45 psi.

Trailer breakaway action

If the trailer separates, the emergency relay valve applies the trailer spring brakes automatically and holds them until air is restored.

Skid recovery action

Release the brake pedal to let locked wheels roll and regain steering; hard braking on slippery surfaces reduces control and increases jackknife risk.

Air compressor drive

The compressor is belt or gear driven from the engine and is cooled and lubricated by the engine; a loose or glazed belt reduces build-up rate.

Air dryer function

The air dryer removes moisture and oil vapour from compressed air; the desiccant cartridge is replaced periodically or when the purge cycle fails.

Relay valve purpose

A relay valve on trailers and long-wheelbase vehicles supplies reservoir air close to the brake chambers, speeding application and release.

Quick-release valve purpose

It rapidly exhausts air from the brake chambers when the brake is released, speeding release and preventing brake drag on long wheelbases.

Spring brake caging

Before removing or servicing a spring brake chamber, the spring must be manually caged (compressed); the chamber must never be opened.

Automatic slack adjuster

A device that self-adjusts during normal brake applications to maintain correct pushrod stroke; it should not be manually shortened to hide wear.

Foot valve function

The treadle valve meters air to the service chambers in proportion to pedal travel, so partial pedal gives partial braking rather than on/off.

Pushrod stroke limit

Each chamber size has a maximum stroke, commonly about 51 mm (2 in) for a Type 30 long stroke; always use the limit stamped on the chamber.

Lining wear limit

Replace bonded lining below 6 mm (1/4 in) thickness, or riveted lining worn to within 1.6 mm (1/16 in) of the rivet heads.

Drum inspection points

Check drums for cracks, heat checking, deep scoring, and out-of-round; a drum beyond maximum diameter or badly scored must be replaced.

Leak detection method

Locate air leaks with soap solution or an ultrasonic detector; common points are glad hands, fittings, chambers, and the dryer purge valve.

Reservoir draining

Drain air reservoirs daily or per schedule to remove moisture and oil; failure to drain causes corrosion, frozen lines in winter, and valve malfunction.

Post-repair testing

After any brake work, repeat the full pre-trip air brake tests (leak, low-pressure warning, spring brake, build-up) before returning the vehicle to service.