MD CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system by drawing in outside air and forcing it into the storage reservoirs, typically through a governor-controlled cycle.

Valve mounted on or near the compressor that senses reservoir pressure and turns the compressor's pumping cycle on at a low cut-in setting and off at a high cut-out setting.

The upper pressure setting at which the governor stops the compressor from pumping, commonly around 120 to 125 psi in a typical heavy-vehicle system.

The lower pressure setting at which the governor restarts the compressor, commonly around 100 psi, so pressure recovers before it falls near the warning threshold.

Device between the compressor and the reservoirs that removes moisture and oil vapor from compressed air, usually with a desiccant cartridge and a purge cycle, helping prevent internal corrosion and winter freeze-ups.

Storage tank that holds compressed air for braking. The wet tank receives air first and collects moisture; dry tanks supply the brake circuits and are protected by one-way check valves.

Manual or automatic valve at the bottom of a reservoir used to release accumulated water and sludge; manual valves must be opened daily during the pre-trip inspection and until only clean air escapes.

One-way valve that allows air to flow in one direction only, keeping pressure from bleeding back out of a reservoir when the compressor stops or another circuit loses pressure.

Spring-loaded relief valve on a reservoir that opens automatically at a preset pressure, usually about 150 psi, to release air and prevent tank rupture if the governor or compressor malfunctions.

Hose coupling lines between truck and trailer; the supply or emergency line charges the trailer reservoirs, while the service line carries the air signal that applies the trailer brakes.

Interlocking coupler fittings that connect the tractor's supply and service air lines to the trailer; they must be sealed, locked, and checked for leaks during hookup.

Valve that seals off the tractor's air supply and applies the trailer brakes if the trailer breaks away or a line ruptures, so the rig can still be stopped and the tractor is not drained of air.

Treadle valve the driver presses to apply service brakes; it meters air pressure to the brake chambers in proportion to how far and how hard the pedal is pushed.

The process of sending air pressure through the foot valve to the brake chambers, where it pushes a diaphragm and pushrod to apply the foundation brakes; more pedal travel means more pressure.

The process of letting air exhaust from the brake chambers when the pedal is released, allowing springs to retract the pushrods and free the wheels; a full release should be heard as a distinct exhaust sound.

Cylinder that converts air pressure into mechanical force; a service chamber pushes the slack adjuster when air is applied, and a spring chamber holds a powerful spring for parking and emergency braking.

Spring-apply, air-release chamber that mechanically applies the brakes when air pressure is removed, providing parking braking and automatic emergency braking if system pressure is lost.

Rod that transfers force from the brake chamber diaphragm to the slack adjuster; excessive free stroke or a pushrod that does not move or return signals a brake defect.

Lever that links the pushrod to the brake cam shaft and takes up clearance as linings wear; it must be adjusted so the pushrod stroke stays within the manufacturer's limit.

Setting of the brake linkage so the pushrod travels only the distance specified for that chamber type; too long a stroke means poor braking and must be corrected before driving.

The wheel-end hardware that actually creates friction and stops the vehicle, including brake shoes or pads, drums or discs, and the cam or wedge that spreads the shoes.

Audible and visual alert that must activate before pressure drops below about 60 psi, warning the driver that braking capability is being lost and that a stop must be made safely.

The audible buzzer and visible lamp that together signal low air pressure; both must work, and a driver should test them during the pre-trip inspection.

Dash instrument showing reservoir pressure in psi; the driver watches it to confirm the compressor is keeping up and to notice pressure loss while driving.

Automatic or deliberate braking used when normal service brakes fail or pressure is lost, typically by spring brakes applying as air bleeds off or by using the parking brake control.

Dash control that releases air from the spring brake chambers to apply the parking brakes when pulled out, and admits air to release them when pushed in; it is usually identified by a yellow diamond knob.

Technique of applying the parking brake control gradually, rather than fully, to slow the vehicle with spring brakes if service brakes fail, avoiding a sudden lockup and skid.

Required walk-around and in-cab check of air brake condition, including air pressure build-up, warning devices, leaks, slack adjuster stroke, and brake performance before driving.

Check made with the engine off, brakes released, and full pressure in the system; pressure should not drop more than the manufacturer's limit over a set time, revealing leaks in the charged system.

Check made with the brakes fully applied and held, engine off; the gauge is watched for excessive pressure drop, which indicates leaks in the applied side of the system.

Check that the compressor raises pressure from a low reading to the governor cut-out within a reasonable time, showing the charging system can keep pace with brake use.

The measured drop in psi per minute during a leak test; exceeding the allowed rate means the vehicle must not be driven until the leak is repaired.

Loss of braking power from overheating, common on long downgrades when brakes are dragged continuously; controlled by gearing down and using brakes in short, firm applications.

Downgrade technique of applying the brakes firmly to reduce speed by a set amount, then releasing them to cool, repeated as needed while in a low enough gear.

Using a lower gear so the engine and driveline resist the vehicle's speed on a downgrade, reducing reliance on the service brakes and helping prevent fade.

Total distance needed to stop, made up of perception and reaction distance plus braking distance; with air brakes, lag in pressure build-up adds time before full braking force arrives.

The brief delay between pressing the pedal and pressure reaching the brake chambers, during which the vehicle continues at speed; it lengthens stopping distance, especially in long combinations.