Ontario Air Brake Z Test key terms

The foot-operated brake system used for normal stopping and slowing. In an air brake vehicle it applies air pressure to the brake chambers at all wheels when the treadle valve is pressed.

A mechanical spring-applied brake that holds a parked vehicle and stays engaged without air pressure. It is released by air pressure or by a manual release mechanism, depending on the system design.

The backup stopping function that applies automatically when system air pressure drops below a safe level. It usually works through spring brakes and a dash valve, allowing a controlled stop rather than a sudden lockup.

A brake chamber containing a powerful spring that applies the brake when air pressure is released. Air pressure compresses the spring to release the brake, so a loss of air causes automatic application.

The cylinder that converts air pressure into mechanical force to apply a brake. A service chamber acts only on service air, while a spring brake chamber also contains the parking spring.

The linkage between the brake chamber pushrod and the brake shoe that transmits force and allows adjustment for lining wear. Excessive pushrod travel signals the need for adjustment or repair.

The distance the brake chamber pushrod extends when the brake is applied. Too much travel means poor braking and must be corrected by adjusting the slack adjuster or repairing the brake.

A self-adjusting slack adjuster that takes up lining wear as the brake is used. It still needs regular inspection, because it cannot compensate for worn drums, broken parts, or a badly out-of-adjustment brake.

The foot valve that the driver presses to apply the service brakes. It meters air pressure to the brake chambers in proportion to pedal travel, giving the driver control over braking force.

A valve mounted near the brakes that speeds up application and release by sending air directly to the brake chambers. It is controlled by a small signal from the treadle valve rather than by full air flow through the cab.

A valve that exhausts air from the brake chambers close to the wheels when the brake is released. This shortens release time and helps prevent brake drag on long vehicles.

A valve with two inlets and one outlet that passes whichever inlet has the higher pressure. It is used to let either of two air sources operate the same circuit, such as service or emergency air.

The engine-driven pump that produces compressed air for the brake system and other air devices. It is cooled and lubricated by the engine and feeds the reservoirs through a governor-controlled cycle.

The device that controls compressor output by unloading the compressor when reservoir pressure reaches the cut-out setting and reloading it at the cut-in setting. It keeps system pressure within a safe operating range.

The reservoir pressure at which the governor stops the compressor from pumping, typically around 120 to 145 psi depending on the system. Pressure above this point is relieved by the safety valve.

The lower reservoir pressure at which the governor restarts compressor pumping after cut-out. The difference between cut-in and cut-out is the compressor cycle range and should be checked during inspection.

A pressure relief valve on the air reservoir that opens if pressure rises above a set limit, protecting the system from overpressure. It is a safety device and must never be plugged or adjusted.

A device that removes moisture and oil from compressed air before it enters the reservoirs. It reduces corrosion and freeze-up in valves and lines and usually contains a replaceable desiccant cartridge and purge valve.

A storage tank that holds compressed air for braking and auxiliary use. Reservoirs are drained regularly to remove water and are protected by check valves so one circuit cannot drain another.

The first reservoir that receives air from the compressor, where most moisture and oil condense. It is drained most often and is separated from the dry tanks by a check valve.

A valve at the bottom of a reservoir used to release accumulated water and oil. Manual drain valves are opened during inspection, while automatic drain valves cycle on their own.

A visual and audible warning that activates when system pressure falls below a safe level, commonly about 60 psi. It alerts the driver before spring brakes apply automatically and must be tested daily.

The dash instrument showing reservoir pressure in the brake system. A vehicle with dual circuits normally has two gauges or a dual-needle gauge so each circuit can be monitored separately.

A gauge that shows the air pressure being delivered to the brake chambers when the brake is applied. It helps the driver judge how hard the brakes are being applied and confirm proper valve operation.

A valve that closes the air lines to a trailer when the trailer breaks away or the lines separate. It keeps the tractor's air supply from escaping and causes the trailer brakes to apply.

A coupling that connects the tractor's service and emergency air lines to the trailer. The two couplings are different shapes so the lines cannot be crossed when hoses are connected.

The hoses that carry service and supply air between tractor and trailer. They must be supported, free of chafing and leaks, and connected to the correct glad hands before moving.

The emergency or supply air line that charges the trailer reservoirs and holds off the trailer spring brakes. Loss of pressure in this line causes the trailer brakes to apply automatically.

The air line that carries the driver's brake application signal from the tractor to the trailer. It works with the relay valve on the trailer to apply the trailer service brakes.

A system with two separate air circuits, usually one for the front axle and one for the rear, so a failure in one circuit still leaves some braking. Each circuit has its own reservoir and warning.

The systematic check of the air brake system before driving, covering leaks, pressure build-up, warning devices, and brake adjustment. It is a legal and safety requirement for commercial drivers.

A test with the brakes released that checks whether air pressure drops too quickly in the system. Excessive drop indicates a leak that must be found and repaired before the vehicle is driven.

A test performed with the service brake held on to check for leaks in the applied circuit and brake chambers. A steady pressure loss beyond the allowed limit means the vehicle must be taken out of service.

A check of how quickly the compressor raises reservoir pressure from a low point to the cut-out setting. Slow build-up can indicate a worn compressor, a leak, or a restricted line.

A measurement of pushrod travel or slack adjuster angle to confirm the brakes are properly adjusted. Brakes out of adjustment must be corrected before the vehicle is operated.

The loss of braking power caused by heat, usually from repeated or prolonged heavy braking. It is managed by using gears and controlled braking rather than riding the brakes on long downgrades.

A device such as an engine brake or exhaust brake that slows the vehicle without using the service brakes. It reduces heat and wear on the brakes and is used on long or steep downgrades.

A technique of braking firmly to reduce speed, then releasing the brake and letting the engine retarder hold the lower speed. It gives the brakes time to cool and helps prevent fade on downgrades.

A technique of applying the brakes firmly to a full stop, then releasing them fully before the vehicle moves again. It is used in some emergency or low-traction situations, not for normal downhill control.

A loss of control in which the tractor and trailer fold at the coupling, often caused by locked wheels or overbraking. Smooth braking and proper trailer brake timing help prevent it.

A loss of traction in which the tires slide instead of rolling, reducing steering and stopping control. Air brakes applied too hard on a slippery surface can cause a skid.

The short delay between pressing the brake pedal and the brakes actually applying, caused by the time air takes to travel through the system. Drivers must allow for it when judging stopping distance.

The total distance travelled from perceiving a hazard to a full stop, including perception, reaction, brake lag, and braking distance. It increases with speed, weight, and poor road conditions.