Saskatchewan Air Brake key terms

The air circuit a driver uses during normal driving to slow or stop the vehicle. It applies pressure gradually through the foot valve, so braking force can be varied from light to full application.

A spring-applied, air-released circuit that holds a parked vehicle and applies automatically if air pressure is lost. It is released only by building enough air pressure to compress the springs.

The backup function that stops the vehicle when service brake pressure fails. On modern vehicles it works through automatic spring brake application rather than a separate manual circuit.

The air pressure delivered to a brake chamber at a given moment, which determines how hard that brake applies. It is read on a gauge or measured at a test fitting during diagnostics.

A storage tank that holds compressed air for the brake system. Tanks are divided into wet, primary, and secondary sections so that a leak or failure in one circuit does not drain the others.

The first reservoir after the compressor, where air cools and moisture and oil drop out before the air reaches the other tanks. It is drained most often because it collects the most contamination.

The reservoir and brake circuit that normally supplies the rear (or a designated split) brakes. It is protected by a one-way check valve so the secondary circuit can still supply air if the primary fails.

The reservoir and brake circuit that normally supplies the front (or the other designated split) brakes. It remains charged when the primary circuit fails, allowing a reduced but controlled stop.

A one-way valve that lets air flow in one direction only. In brake systems it keeps pressure from escaping back out of a reservoir, preserving air in the circuit that is still working.

The engine-driven pump that produces the compressed air supply for the brake system. It is cooled and lubricated by the engine and is controlled by a governor so it does not pump continuously.

A valve that senses reservoir pressure and switches the compressor between pumping and unloading. It sets the cut-in and cut-out pressures and also triggers purging of the air dryer.

The upper pressure at which the governor stops the compressor from pumping, typically around 120 to 125 psi on heavy vehicles. Pressure above this point is released by the safety valve instead.

The lower pressure at which the governor restarts the compressor after it has unloaded, commonly about 100 psi. The gap between cut-in and cut-out is the normal operating pressure range.

A spring-loaded relief valve on a reservoir that opens if pressure rises above a safe limit, usually about 150 psi. It protects tanks and lines from over-pressurization if the governor fails.

A device between the compressor and the wet tank that removes water vapor and oil from the air. Most units are purge-type and are cycled by the governor each time the compressor unloads.

An optional device that injects alcohol vapor into the air system to lower the freezing point of moisture. It reduces winter ice formation in valves and lines but does not replace regular draining.

A valve fitted to a reservoir that expels collected water and oil automatically, often each time the compressor cycles. It still needs periodic inspection because it can clog and stop draining.

A hand-operated valve at the bottom of a reservoir used to blow off accumulated water and oil. Draining daily, especially in cold weather, prevents freeze damage and contaminated air.

The treadle-operated valve that meters air from the reservoirs to the brake chambers according to how far the pedal is pushed. It also provides a feel of application to the driver.

A valve mounted near the brakes that uses a small control signal to admit a large volume of air directly from a nearby reservoir. It speeds up application and release on long wheelbases.

A valve placed close to a brake chamber that exhausts air quickly when the pedal is released. It shortens release time and helps prevent brake drag on long vehicles.

The cylinder where air pressure acts on a diaphragm or piston to push the pushrod and apply the brake. Its size and stroke determine the force delivered to the slack adjuster.

A chamber combining a service section with a strong spring that applies the brake when air is released. It provides parking and emergency braking and requires careful caging before service work.

A mechanical bolt used to compress and hold the parking spring so the brake releases when there is no air pressure. It is a temporary service aid, not a substitute for repair.

A valve that prevents service brake pressure and spring brake force from being applied to the same chamber at the same time. It avoids mechanical overload and possible brake component damage.

The lever that connects the pushrod to the brake camshaft and converts linear pushrod travel into rotary brake action. Its angle and length affect both force and the correct adjustment range.

The rod that transfers force from the brake chamber diaphragm to the slack adjuster. Its travel is the measurement used to judge whether a brake is properly adjusted.

The distance the pushrod moves during a full brake application, measured with a ruler at a set pressure. Excessive stroke means the brake is out of adjustment and must be corrected before driving.

A slack adjuster that takes up clearance during normal brake use without manual adjustment. It still must be checked, because a worn or damaged unit may not adjust and can leave excessive stroke.

Setting the clearance between brake linings and drum so pushrod stroke stays within the manufacturer's limit. Poor adjustment causes weak braking, overheating, and longer stopping distances.

Loss of braking force caused by heat, usually from dragging brakes or repeated hard stops. It may also come from wet linings that recover after the brakes dry through use.

A light and buzzer that activate when reservoir pressure falls below a set threshold, usually about 60 psi. It warns the driver to stop before spring brakes apply automatically.

The dash instrument showing pressure in the primary and secondary circuits. The driver watches it to confirm buildup, normal operating range, and that pressure is not falling while driving.

A check for leaks made with the engine off and brakes released, watching the gauge for a pressure drop over a set time. Excessive loss means the vehicle must not be driven until repaired.

A leak check performed with the service brake fully applied and held, noting gauge drop over a specified period. It finds leaks in the service circuit that a released-brake test would miss.

The amount of pressure lost per minute during a leak test, compared with the limit in the vehicle's inspection standard. A rate above the limit indicates a defect requiring repair.

The time needed for the compressor to raise reservoir pressure from cut-in to cut-out with the engine at operating speed. An excessive time points to a weak compressor or a leak.

A controlled stop made when service brakes fail or pressure is lost, using spring brake application and engine braking. The driver should keep steering, warn others, and stop away from traffic if possible.

Repeated on-off brake applications that use air faster than the compressor can replace it. It can drop pressure to the point that spring brakes apply, so steady pressure is preferred.