NY CDL Air Brakes key terms

Engine-driven pump that builds and maintains air pressure in the brake system reservoirs. It cycles on and off as pressure drops and rises, and its governor controls when it pumps.

Valve device that senses reservoir pressure and turns the compressor's pumping action on at a low set point and off at a high set point, usually around 100 to 125 psi on a typical system.

Storage tank that holds compressed air for braking. A tractor-trailer has at least one supply tank and separate tanks for the front and rear brake circuits so a leak cannot drain every circuit at once.

The wet tank in a dual system. It receives air directly from the compressor, collects moisture and oil, and feeds the primary and secondary circuits through one-way check valves.

Valve that lets air flow in one direction only. It keeps air from backflowing out of a circuit, so a leak or failure in one part of the system cannot drain the rest.

Brake system split into two independent circuits, typically primary and secondary, each with its own reservoir and protected by check valves, so a single failure still leaves some braking ability.

One of the two split brake circuits. In a typical truck it operates the rear (drive) axle brakes and is fed by its own reservoir through a check valve.

The other split brake circuit, usually operating the front (steering) axle brakes. It has its own reservoir and check valve so it keeps working if the primary circuit fails.

The brake pedal assembly that meters air from the reservoirs to the brake chambers in proportion to how far the driver pushes it, allowing light or hard brake application.

Cylinder where air pressure pushes a diaphragm and pushrod to apply the foundation brakes. Its size and stroke determine how much force reaches the slack adjuster.

Lever on the brake camshaft that converts pushrod movement into brake shoe application and automatically takes up clearance as linings wear, keeping pushrod stroke within limits.

Distance the brake chamber pushrod travels when the brakes are fully applied. Excessive stroke means poor adjustment and must be corrected before the vehicle is driven.

Brake chamber section with a strong spring that applies the brakes when air pressure is released, used for parking and as an emergency brake if air pressure is lost.

Dash valve, usually a yellow diamond knob, that releases air from the spring brakes to apply them for parking and supplies air to release them for driving.

Valve that prevents service brake air pressure from being applied on top of spring brake force at the same time, avoiding excessive force and damage to brake components.

Dash instrument showing reservoir pressure, usually with separate needles for the primary and secondary circuits so the driver can spot a failing circuit.

Gauge that shows how much air pressure is being delivered to the brakes during a brake application, helping the driver judge braking effort on grades.

Required warning, typically a red light and buzzer, that activates when reservoir pressure drops to about 60 psi or below, alerting the driver before braking ability is lost.

The pressure at which the governor restarts compressor pumping, commonly around 100 psi. Below this, pressure is not being replenished and the driver should watch the gauge.

Check for air leaks with the engine off and brakes released, watching the gauge for pressure drop over time. Any steady loss indicates a leak to find and fix.

Leak check made with the brakes fully applied and held, engine off, to see whether pressure falls. It detects leaks in the service brake side that a static test may miss.

Pre-trip and periodic check of compressor build-up, governor cut-in and cut-out, leak rates, low pressure warning, spring brake operation, and pushrod stroke before driving.

Time needed for the compressor to raise reservoir pressure from about 85 to 100 psi at normal engine speed. Excessive time points to a weak compressor or a leak.

Device between the compressor and the first reservoir that removes moisture and oil from compressed air, protecting valves and tanks from corrosion and freeze-up in cold weather.

Valve at the bottom of each reservoir used to blow off accumulated water and oil, usually checked daily during the pre-trip inspection.

Braking available after a failure, such as spring brakes applying automatically when air pressure is lost, or the remaining circuit stopping the vehicle in a dual system.

Loss of braking power from overheating brakes, common on long downgrades when brakes are used continuously instead of engine braking and a controlled lower gear.

Technique of braking firmly to reduce speed by about 5 to 6 mph, then releasing to let brakes cool, repeated down a grade instead of riding the brakes.

Loss of the air source, shown by pressure that will not build or keeps falling. The driver should stop safely, since spring brakes will soon apply as pressure drops.

Condition where a brake does not release because air cannot escape or the pushrod is jammed, causing drag, heat, and possible fire; it must be repaired before driving.

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

Pressure-relief valve on the supply reservoir that opens if pressure rises too high, protecting tanks and lines from over-pressurization if the governor fails.

Valve mounted near the brakes that uses a small control signal from the foot valve to send a large volume of air quickly to the brake chambers, speeding application and release.

Valve that exhausts air from brake chambers close to the wheels rather than back through the foot valve, so brakes release faster and drag is reduced.

Valve that seals the tractor's air system if the trailer breaks away or a trailer line fails, keeping enough air in the tractor to stop it.

Two hoses, service and emergency (supply), connecting tractor and trailer. The emergency line charges trailer tanks and triggers trailer spring brakes if it separates.

Reservoir pressure at which the governor stops the compressor from pumping, commonly around 125 psi. Pressure above this range suggests a governor or safety valve problem.