MO CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system, typically cycling between a cut-out and cut-in pressure to keep the tanks charged while the vehicle runs.

Valve that monitors system pressure and signals the compressor to stop pumping at cut-out pressure and resume pumping at cut-in pressure, controlling the working pressure range.

The upper pressure limit, commonly around 120 to 125 psi in many systems, at which the governor unloads the compressor so tank pressure stops rising.

The lower pressure limit, often about 100 psi, at which the governor lets the compressor pump again after pressure has dropped through normal use.

Sealed reservoir that holds compressed air for braking and other uses; tanks are drained of moisture and checked for damage and secure mounting during inspection.

Petcock or cable-operated valve at the bottom of an air tank used to release accumulated water and oil, which should be drained daily or as the manufacturer specifies.

Device that removes moisture and contaminants from compressed air before it reaches the storage tanks, helping prevent freeze-ups and corrosion in valves and lines.

Optional unit that injects alcohol vapor into the air system to lower the freezing point of moisture, reducing winter ice problems in valves and lines.

Pressure-relief valve on the air tank that opens automatically to release air if system pressure climbs above a safe level, protecting tanks and components from overpressure damage.

The pedal-operated valve that meters air from the tanks into the service brake lines, applying braking force in proportion to how far the pedal is pressed.

Cylinder that converts air pressure into mechanical pushrod movement, moving the slack adjuster and applying the foundation brakes at the wheel.

Rod extending from the brake chamber that transfers force to the slack adjuster; excessive free travel indicates the brakes need adjustment.

Lever connecting the pushrod to the brake camshaft, converting chamber pushrod stroke into rotation that applies the brakes, and providing a means to adjust brake clearance.

Slack adjuster that adjusts itself during normal brake use; it still requires inspection and correct pushrod stroke measurement, since it can fail to adjust.

Brake chamber section with a strong spring that applies the brakes when air pressure is lost or when the parking control is set, providing emergency and parking braking.

Dash valve that releases air to let spring brakes apply for parking, and supplies air to release them; it is pushed in to release and pulled out to park.

Valve that prevents service brake air and spring brake force from applying at the same time on the same chamber, avoiding excessive mechanical stress and brake damage.

Valve mounted near the brakes that speeds up application and release by sending tank air directly to the brake chambers when signaled by the foot valve.

Valve that exhausts air quickly from brake chambers when the pedal is released, shortening brake release time and reducing lag between pedal and wheel response.

Dash light and buzzer that activate when system pressure drops below a set level, commonly around 60 psi, warning the driver of an unsafe braking condition.

Air line, usually color-coded, that carries air from the tractor to the trailer to charge its tanks and to signal loss of pressure for emergency braking.

Air line that carries the application signal from the tractor foot valve to the trailer, causing the trailer brakes to apply when the pedal is pressed.

Coupling that connects tractor and trailer air lines; mismatched or crossed glad hands can cause brake or supply problems and must be checked during inspection.

Valve that closes the tractor's air supply to the trailer if the trailer breaks away or a line separates, preserving tractor air and applying trailer emergency brakes.

Situation in which a trailer separates from the tractor; the trailer's emergency system must automatically apply its spring brakes to stop it.

Check for air loss with the system charged and brakes released, watching the gauge over time; excessive drop indicates leaks that must be fixed before driving.

Check for leaks with the brakes held on and the system charged; a pressure drop beyond the allowed rate signals leaks in the applied brake circuit.

The amount of pressure lost per minute during a leak test; the acceptable limit depends on vehicle type and whether brakes are released or applied.

Procedure of reducing pressure until the warning activates and the governor cuts the compressor back in, confirming both safety devices work at proper pressures.

Distance the brake chamber pushrod travels when the brakes are applied; measuring it against the manufacturer's limit shows whether brakes are properly adjusted.

Setting of the foundation brake linkage so pushrod travel stays within limits; out-of-adjustment brakes reduce stopping power and are a common inspection failure.

The delay between pressing the pedal and the brakes actually applying, caused by air traveling through lines; it makes stopping distance longer than in hydraulic systems.

Total distance traveled from perception to full stop, including perception, reaction, brake lag, and braking distance; it grows with speed and weight.

Loss of braking power from overheated drums and linings, often after repeated or prolonged braking, requiring the driver to slow down and use lower gears.

Technique of braking firmly to reduce speed, then releasing and letting brakes cool while continuing downhill, rather than riding the brakes continuously.

Auxiliary braking device such as an engine brake or exhaust brake that slows the vehicle without using the service brakes, helping prevent overheating on long downgrades.

Controlled stop using spring brakes or other means when service brakes fail or pressure is lost; the driver should keep the vehicle under control and stop safely.

Repeatedly applying and releasing the brake pedal, which uses air faster than the compressor can replace it and can lead to dangerously low pressure.