NJ CDL Air Brakes Test key terms

Engine-driven pump that draws in outside air, compresses it, and pushes it into the storage tanks to build and maintain system pressure. It is normally cooled and lubricated by the engine and cycles on and off as pressure falls.

Pressure-sensitive valve mounted on or near the compressor that starts and stops compression. It cuts the compressor out at the cut-out pressure and back in at the cut-in pressure, keeping tank pressure within a safe range.

The upper pressure setting, typically around 120 to 125 psi, at which the governor unloads the compressor so it stops adding air. Pressure then holds or slowly drops until the cut-in point is reached.

The lower pressure setting, usually about 100 psi, at which the governor lets the compressor pump again. A gauge that never reaches cut-out or drops far below cut-in signals a problem.

Canister, usually mounted between compressor and tanks, that removes water vapor and oil from compressed air. Most have a desiccant cartridge and a purge cycle; a failed dryer lets moisture reach valves and tanks where it can freeze.

Storage tank that holds compressed air for the brake system. Tanks are drained of water and oil through drain cocks, and each tank has a check valve so air cannot flow backward out of it.

Small manual or automatic valve at the bottom of a reservoir used to blow off accumulated water and oil. Manual cocks must be opened during the pre-trip inspection and at the end of each day of use.

One-way valve that allows air to flow in only one direction. In the brake system it keeps air from escaping back through the compressor or from one reservoir to another, protecting stored pressure.

Pressure-relief valve on the air tank that opens automatically if the governor fails and pressure climbs too high, usually around 150 psi. It protects tanks and lines from bursting from overpressure.

The brake pedal assembly that the driver pushes to apply the service brakes. It meters air pressure to the brake chambers in proportion to how far the pedal is pressed, and releases air when the pedal is let up.

Cylinder at each wheel where air pressure pushes a diaphragm and push rod to apply the brake. In a spring brake chamber the same housing also contains a powerful spring for parking and emergency braking.

Link between the brake chamber push rod and the brake shoe that converts push-rod stroke into shoe movement. Too much free play or a push rod that travels too far means the brakes need adjustment or repair.

Drum brake in which an air-driven push rod rotates an S-shaped cam that spreads the brake shoes against the drum. It is the most common foundation brake on heavy trucks and needs periodic slack adjustment.

Valve mounted near the brake chambers that exhausts air quickly when the pedal is released, so brakes release without waiting for air to travel back to the foot valve. It speeds release and reduces brake drag.

Valve that uses a small control signal from the foot valve to send a large volume of air from a nearby reservoir to the brake chambers. It shortens apply time and improves braking response on long vehicles.

Dashboard gauge that shows how much air pressure is being sent to the brake chambers when the brakes are applied. It helps the driver judge how hard the brakes are working and confirm they are releasing.

Dashboard gauge showing the air pressure available in the system reservoirs. On a dual system there are normally two needles, and the driver must watch both to know whether pressure is being lost.

Buzzer or light that activates when system pressure drops below a set level, commonly about 60 psi. It warns the driver to stop before the spring brakes apply automatically from loss of air.

System with two separate air circuits, typically one for the front axle and one for the rear, each with its own reservoir and check valve. A failure in one circuit still leaves the other able to stop the vehicle.

The air circuit, usually serving the rear service brakes, that is charged first and holds pressure for normal braking. Its reservoir is protected by a check valve so the other circuit cannot drain it.

The second air circuit, commonly serving the front brakes, kept separate from the primary circuit. If the primary fails, the secondary still provides braking, though stopping distance will be longer.

Brake chamber section held off by air pressure and applied by a heavy spring when air is released. It acts as the parking brake when the button is pulled and applies automatically if system pressure is lost.

Dash valve, usually a yellow diamond-shaped knob, that releases air from the spring brake chambers to apply the parking brakes, and supplies air to release them. Pushing it in with low pressure will not release the brakes.

Valve that prevents spring brake force and service brake force from being applied at the same time on the same chamber, which could damage the brake mechanism. It protects the foundation brakes during combined applications.

Valve that closes the air lines between tractor and trailer when they separate or when the trailer supply line loses pressure, sealing air in the tractor and applying the trailer spring brakes.

Air line, normally color-coded, that charges the trailer reservoirs and signals the tractor protection valve. A break or disconnect in this line triggers a loss of trailer air and applies the trailer emergency brakes.

Coupler that joins the tractor and trailer air lines. It must be locked and sealed; a leaking or unseated glad hand causes pressure loss and can trigger the tractor protection valve.

Device that injects alcohol vapor into the air system to lower the freezing point of moisture, helping prevent ice in valves and lines in cold weather. It supplements, but does not replace, regular draining of tanks.

Required walk-around check before driving that includes draining air tanks, checking compressor build-up, listening for leaks, and testing warning devices and brakes. It confirms the air brake system is safe before the vehicle moves.

Step in the pre-trip inspection where each reservoir drain cock is opened to blow out water and oil. If a lot of liquid comes out, the air dryer or compressor may need service.

Test of how fast the compressor raises pressure from a low point, commonly from 85 to 100 psi within about 45 seconds at normal idle. Slow build-up points to a weak compressor, leaks, or a slipping belt.

Check for air loss with the engine off and brakes released, then applied, watching the gauges for a drop beyond the allowed rate. It finds leaks in lines, valves, and chambers before they become a road failure.

Leak test done with the engine off, system fully charged, and service brakes released. The driver watches the gauges for a specified time; a steady drop in pressure means air is escaping somewhere in the system.

Leak test performed with the engine off, full pressure, and the brake pedal held down firmly. The gauge should hold steady; a drop indicates a leak in the applied side, such as a chamber or hose.

Test in which the driver pumps the brakes with the engine off and confirms the low pressure warning buzzer and light come on before pressure falls below about 60 psi. It proves the driver will be warned of air loss.

Check that the parking brake knob pops out and the spring brakes apply when pressure drops, then that the knob can be pushed in only after pressure is restored. It confirms the emergency and parking brake function.

Low-speed check that the service brakes stop and hold the vehicle without pulling or unusual noise, done after the air system checks. It confirms the foundation brakes and air application are working together.

Setting of the slack adjusters so push-rod stroke stays within the manufacturer's limit. Too much stroke means the chamber runs out of travel, giving weak braking and a possible out-of-adjustment violation.

Distance the brake chamber push rod travels when the brakes are fully applied. It is measured against the manufacturer's limit; excessive stroke shows the brakes are out of adjustment or a chamber is failing.

Braking technique for slippery or emergency conditions where the driver applies the brakes fully, then releases them when wheels lock, repeating as needed. It restores steering and helps avoid a long skid.

Applying the brakes as hard as possible just short of locking the wheels, then holding that pressure. It gives maximum stopping force while keeping steering control, especially on dry pavement.

Condition where a braked wheel stops turning and slides, causing loss of steering and longer stopping distance. It is more likely with empty trailers, light loads, or slippery surfaces and calls for releasing and reapplying the brakes.

Loss of braking power from overheated drums and shoes, common after repeated or prolonged hard braking on downgrades. The driver should use a lower gear and engine braking rather than riding the brakes.

Downgrade technique of braking firmly to reduce speed, then releasing the brakes to let them cool, repeating as needed. It avoids continuous light brake pressure that overheats the brakes and causes fade.

Using a lower transmission gear so the engine's compression slows the vehicle, reducing reliance on the service brakes. It is the main protection against brake fade on long or steep downgrades.

Actions taken when air pressure is lost or brakes fail: keep both hands on the wheel, downshift, use the parking/spring brakes gradually, and look for a safe place to stop. Panic pumping and shutting off the engine make things worse.

Stopping method when service air is gone: the spring brakes apply automatically as pressure bleeds off, and the driver steers to a safe spot while the vehicle slows. Pulling the parking knob can speed the application.

Rupture or disconnect of an air line that causes rapid pressure loss and possible automatic spring brake application. The driver should stop as soon as safely possible rather than continue to the destination.

Systematic search for the source of pressure drop by checking gauges, listening for leaks at lines, valves, glad hands, and chambers, and using soapy water. Finding the leak determines whether the vehicle can be driven.

Fault where the gauge stays low or climbs very slowly, caused by a slipping or broken belt, failed compressor, stuck governor, frozen line, or major leak. It must be repaired before the vehicle is driven.

Condition where brakes do not fully release, causing heat, poor performance, and possible fire. It can come from a stuck quick release valve, kinked hose, or misadjusted slack adjuster and should be repaired promptly.

Flexible rubber disc inside a brake chamber that air pressure pushes against to move the push rod. A torn diaphragm leaks air and weakens braking, often showing up as a hiss or a slow pressure drop.