ME CDL Air Brakes key terms

Engine-driven pump that builds and maintains the supply of compressed air stored in the vehicle's reservoirs, cycling on and off as pressure drops below the governor cut-in setting.

Valve mounted on or near the compressor that starts and stops compressor pumping, controlling system pressure between a cut-out (about 120-125 psi) and a cut-in (about 100 psi) setting.

The first storage tank to receive compressor discharge; it collects moisture and oil carryover before air passes to the dry tank and is protected by a drain valve.

Cartridge device between the compressor and the first reservoir that removes moisture and oil vapor from compressed air, usually with a heater and a purge cycle to expel collected water.

Optional device that injects alcohol vapor into the air system to lower the freezing point of moisture, helping prevent ice blockage in valves and lines in cold weather.

Manual or automatic valve at the bottom of each air tank used to blow off accumulated water and sludge; tanks must be drained daily during a pre-trip inspection.

Spring-loaded relief valve on the supply reservoir that vents air at a set pressure (about 150 psi) to protect tanks and lines from over-pressurization if the governor fails.

Valve that permits air to flow in only one direction, keeping pressure from bleeding back out of a reservoir or circuit when supply pressure drops.

The pedal-operated valve that meters air from the reservoirs to the brake chambers in proportion to how far and how hard the driver pushes the treadle.

Cylinder containing a diaphragm and pushrod that converts air pressure into mechanical force to apply the foundation brakes when air enters from the foot or relay valve.

A brake chamber with a heavy spring that applies the brakes mechanically when air pressure is released, providing parking and emergency braking and releasing only when air is restored.

Dash valve, identified by a yellow diamond-shaped knob, that exhausts air from spring brake chambers to apply the parking brake and admits air to release it.

Valve that prevents service brake application air from combining with spring brake force, avoiding excessive mechanical stress on brake components when both systems are applied at once.

Valve mounted near the rear axles that uses a small control signal from the foot valve to quickly deliver a large volume of air to the rear brake chambers, reducing lag time.

Valve that speeds release of air from brake chambers when the driver lets off the pedal, reducing brake lag and helping prevent brake drag on the axle it serves.

Lever connecting the brake chamber pushrod to the brake camshaft; its length setting converts pushrod travel into the correct rotation for brake shoe application and must be properly adjusted.

Distance the brake chamber pushrod extends when the brakes are applied; too much travel means the brakes are out of adjustment and stopping power is reduced.

Slack adjuster that self-adjusts during normal brake use; it still requires inspection, and a mechanic must repair it if travel exceeds limits rather than manually shortening it.

Check of pushrod travel with the brakes fully applied and system pressure at 90-100 psi, comparing measured stroke against the vehicle manufacturer's limit for that chamber type.

Test with the engine off and brakes released that checks whether system air pressure drops faster than the allowed rate, revealing leaks in tanks, lines, and valves.

Test performed with the service brakes fully applied and held for one minute to confirm pressure loss stays within limits, exposing leaks in chambers, diaphragms, and hoses.

Allowable pressure drop measured during leak tests, commonly no more than 3 psi per minute for a single vehicle and 4 psi per minute for a combination with brakes released.

Dash light and buzzer that activate when system pressure falls to about 60 psi or below, warning the driver that braking performance is compromised and a stop is required.

Controlled stop using downshifting, engine braking, and light steady service braking after a low-pressure warning, done off the travel lane before spring brakes fully apply.

Time required for the compressor to raise reservoir pressure from about 85 to 100 psi; a slow rate (over roughly 45 seconds for a combination) signals a compressor or leak problem.

Delay between the driver pressing the pedal and the brakes actually applying, caused by the time air takes to travel through lines and valves; it lengthens stopping distance.

Emergency technique of pressing the brake pedal fully and holding it, releasing only if wheels lock, then reapplying; used on vehicles without antilock brakes.

Braking technique that applies pedal force just short of wheel lockup, keeping tires rolling and preserving steering control while producing maximum available stopping force.

Electronic system that senses wheel lockup and rapidly modulates brake pressure to keep wheels rolling, helping the driver retain steering control during hard braking on slippery surfaces.

System that works with ABS to reduce wheel spin during acceleration by applying brakes or cutting engine torque, improving traction on low-friction surfaces.

Condition in which moisture in air lines or valves turns to ice in freezing weather, blocking air flow and causing brakes to drag or fail to release.

Dash instrument showing reservoir pressure in psi; the driver watches it to confirm the compressor is keeping up and to notice leaks or abnormal cycling.

Dash gauge showing how much air pressure the driver is applying to the brakes, useful for judging braking effort on downgrades and detecting over-application.

Required walk-around and in-cab check covering leaks, drain valves, warning devices, build-up rate, and brake adjustment before a vehicle is placed in service.

Valve that closes off the tractor air supply when a trailer breaks away or a line separates, keeping enough pressure to stop the tractor and apply its spring brakes.

Glad-hand hose that carries air from tractor to trailer to charge trailer reservoirs and release trailer spring brakes; if it separates, trailer brakes apply automatically.

Endorsement required to drive vehicles equipped with air brakes; it is earned by passing a knowledge test and is removed from the license if the driver fails to demonstrate air brake knowledge.

Loss of the air brake restriction removal or endorsement, which occurs when a driver fails the air brake knowledge or inspection portion of the CDL skills test.

License restriction (commonly code L) prohibiting operation of commercial vehicles with air brakes, applied to drivers who took the CDL test in a vehicle without air brakes.