Saskatchewan Air Brake cheat sheet

A compressor builds roughly 100-120 psi (690-825 kPa) in the wet tank; the governor cuts the compressor in at about 85 psi and out at about 105-125 psi depending on the setting.

Pressure must never fall below 100 psi (690 kPa) before the low-pressure warning activates; Saskatchewan commercial vehicles require a warning device that signals before pressure drops below 60 psi (414 kPa).

Spring brakes apply mechanically when air pressure is released from the spring-brake chambers; they release only when at least about 90-100 psi is available to compress the spring.

The tractor protection valve closes automatically when supply pressure drops to about 20-45 psi, sealing the tractor's air supply from the trailer to preserve tractor braking.

A dual-circuit system separates the primary (rear) and secondary (front) circuits so a single failure leaves at least one service brake circuit functional.

Compressor pumping time: with the system at cut-in pressure, a properly functioning system builds from 85 to 100 psi in about 45 seconds at governed engine speed.

Air pressure leakage limit for a single vehicle is 3 psi per minute (static, brakes released) and 4 psi per minute with brakes applied; combination units allow slightly more.

The foot valve (treadle valve) meters air to the service chambers in proportion to pedal travel; it is not an on/off switch, so partial pedal gives partial braking.

Static leak test (unapplied): build to governor cut-out, shut engine off, release brakes, wait one minute; loss must not exceed 3 psi for a single unit or 4 psi for a combination.

Applied leak test: with full pressure and engine off, apply and hold the service brake for one minute; pressure loss must not exceed 4 psi for a single unit or 6 psi for a combination.

Low-pressure warning test: with engine off, pump the brake pedal until the warning light and buzzer activate; they must come on before pressure falls below 60 psi (414 kPa).

Spring brake (emergency) test: with pressure above 90 psi, push in the parking control and check that the spring brakes release; then pull it out and confirm they apply.

Tractor protection valve 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 retain its air.

Compressor build-up test: at fast idle, pressure should rise from 85 to 100 psi within about 45 seconds; a slower rise indicates a worn compressor, slipping belt, or leak.

Visual inspection items: check slack adjuster travel, brake chamber pushrod stroke, lining thickness (minimum 6 mm bonded or 1.6 mm riveted above rivet heads), drum condition, and air line chafing.

The pushrod stroke must be measured with the brakes fully applied and system pressure at least 90 psi; compare against the chamber's adjustment limit stamped on the chamber or in the manual.

Brake fade occurs when overheated drums and linings lose friction; control speed downhill by gearing down and using intermittent firm brake applications, not continuous light pressure.

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

If the low-pressure warning sounds while driving, stop safely as soon as possible; continued driving will cause the spring brakes to apply automatically at about 20-45 psi.

On a long downgrade, select a gear that lets engine braking hold the desired speed; apply the brakes firmly enough to feel a definite retard, then release to allow cooling.

If the spring brakes apply unexpectedly, do not fight them; steer to a safe stop, set the parking control, and diagnose the air loss before moving again.

A trailer breakaway (glad hand separation) triggers the trailer emergency relay valve, which applies the trailer spring brakes automatically and holds them until air is restored.

When skids or jackknife risk exists, release the brake pedal to let wheels roll and regain steering; braking hard on a slippery surface with locked wheels reduces control.

After any emergency stop caused by air loss, do not resume driving until the leak is found and repaired and the system rebuilds to governed pressure.

The compressor, driven by belt or gear from the engine, supplies air; it is cooled and lubricated from the engine and should not be run with a loose or glazed belt.

The air dryer removes moisture and oil vapour; the desiccant cartridge typically requires replacement every 1-2 years or when the purge cycle fails to clear moisture.

The governor senses reservoir pressure and controls compressor loading (cut-out) and unloading (cut-in) to maintain the operating range.

The foot valve meters air to the service chambers; the relay valve on trailers and long-wheelbase vehicles speeds application by supplying reservoir air close to the chambers.

The quick-release valve exhausts air rapidly when the brake is released, speeding release and preventing brake drag on long wheelbases.

Spring brake chambers contain a strong mechanical spring; they must be caged (manually compressed) before removal or service, and the chamber must never be opened.

Slack adjusters transmit pushrod force to the brake cam; automatic slack adjusters self-adjust during normal brake applications and should not be manually adjusted to compensate for worn linings.

The low-pressure warning switch and buzzer/light are separate from the gauge and must be tested; a faulty switch can leave the driver unaware of a pressure loss.

Pushrod stroke limits vary by chamber size; for a typical Type 30 chamber the limit is about 51 mm (2 in) with a long stroke, so always use the limit stamped on the chamber.

Automatic slack adjusters should be checked by measuring stroke at full application; if stroke exceeds the limit, the adjuster or the foundation brake needs repair, not manual shortening.

Brake lining replacement is required when bonded lining is worn to less than 6 mm (1/4 in) or riveted lining is within 1.6 mm (1/16 in) of the rivet heads.

Drums should be checked for cracks, heat checking, and out-of-round; a drum worn beyond its maximum diameter or with deep scoring must be replaced.

Air system leaks are located with soap solution or an ultrasonic leak detector; common points are glad hands, fittings, chambers, and the dryer purge valve.

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

Check compressor belt tension and condition; a slipping belt reduces build-up rate and can cause the low-pressure warning to activate during normal driving.

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