Minnesota CDL Air Brakes Test key terms

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

Valve device that monitors system pressure and turns the compressor's pumping action on at a low cut-in pressure and off at a high cut-out pressure, typically around 100 and 125 psi.

Sealed reservoir that holds compressed air for braking, keeping enough reserve capacity so brakes can still be applied several times if the compressor fails.

Pressure-relief device on the air tank that opens automatically to release air if system pressure climbs above a safe limit, protecting tanks and lines from rupture.

Manually or automatically operated valve at the low point of an air tank used to blow off accumulated water and oil so they do not travel into brake components.

Canister or cartridge unit between the compressor and the tanks that removes moisture from compressed air, reducing corrosion and preventing ice or water from reaching brake valves.

Treadle the driver pushes to send air pressure from the tanks toward the brake chambers; how far it is pushed controls how much pressure reaches the brakes.

Cylinder at each wheel where air pressure pushes a diaphragm and push rod, converting air pressure into the mechanical force that applies the brake shoes or pads.

Linkage arm connecting the brake chamber push rod to the brake shoe or cam; its adjustment sets how much push-rod travel occurs before the brakes actually engage.

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

Dashboard instrument showing the air pressure available in the system; drivers must watch it to confirm the compressor is keeping pressure in the normal operating range.

Gauge that shows how much air pressure is actually being sent to the brakes at that moment, helping the driver judge how hard the brakes are working.

Buzzer and/or light that activates when system pressure drops to an unsafe level, warning the driver that braking ability is being lost and a stop is needed.

System with two separate air circuits, usually a primary and a secondary, so that a leak or failure in one circuit still leaves the other circuit able to stop the vehicle.

One of the two independent air circuits in a dual system, typically supplying the rear brakes, and able to keep working if the other circuit fails.

The other independent air circuit in a dual system, typically supplying the front brakes, providing a backup means of stopping if the primary circuit loses pressure.

Emergency and parking brakes held off by air pressure; when air pressure drops too low, strong springs apply the brakes automatically, bringing the vehicle to a stop.

Dash valve, often a yellow diamond-shaped knob, that releases air from the spring brake chambers to apply the parking brakes and admits air to release them.

Valve that controls how quickly air escapes from spring brake chambers so the spring brakes apply gradually instead of slamming on instantly during an emergency.

One-way valve that allows air to flow in only one direction, keeping air from bleeding back out of a tank or from one circuit into a failed circuit.

Device that adds alcohol vapor to the air system to lower the freezing point of moisture, helping prevent ice from blocking valves and lines in cold weather.

Required walk-around and in-cab check of tanks, lines, slack adjusters, warning devices, and leakage before driving, confirming the brake system is safe and properly adjusted.

Check made with the engine off, brakes released, and full pressure built up; pressure should not drop more than about 2 psi in one minute for a single vehicle.

Check made with the engine off and the brake pedal held down firmly; pressure should not drop more than about 3 psi in one minute for a single vehicle.

In-cab check that the warning buzzer and light come on before pressure falls below about 60 psi, confirming the driver would be alerted to a dangerous pressure loss.

Check that the parking brake knob pops out automatically when system pressure drops to roughly 20 to 45 psi, proving the emergency spring brakes will engage on their own.

Check that the compressor raises pressure from about 85 to 100 psi within a set time, showing the compressor and governor are working properly.

Repeated quick taps of the brake pedal that use up air pressure faster than the compressor can replace it, a practice to avoid on long downgrades.

Downgrade technique of applying the brakes firmly to slow well below the safe speed, then releasing them fully to let them cool before the next application.

Loss of stopping power caused by overheated brake drums, shoes, or pads, often from dragging the brakes too long on a steep descent.

Total distance traveled from the moment a hazard is seen until the vehicle stops, combining perception, reaction, and actual braking distance.

Relationship in which braking distance grows much faster than speed, so doubling speed roughly quadruples the distance needed to stop.

Hard, controlled brake application in a crisis, keeping steady pressure and steering while avoiding a panicked full-lock stop that can cause a skid or jackknife.

Condition where the parking brake control is left partly applied while the foot brake is used, causing conflicting forces and possible damage to brake components.

Additional license qualification required to drive vehicles equipped with air brakes, obtained by passing the knowledge and skills tests for this system.