CDL Air Brakes key terms

An engine-driven pump that draws in outside air, squeezes it, and pushes it into the storage tanks. It keeps enough pressure on hand to apply the brakes many times.

A valve on the compressor that switches pumping on and off. It stops the compressor at a set high pressure and restarts it at a lower pressure, holding system pressure in a safe range.

A steel tank that stores compressed air for braking. The reserve lets the driver apply the brakes several times, and stop safely, even if the compressor quits.

The first tank air reaches after leaving the compressor. It traps moisture, oil, and dirt carried in the airstream before the air moves on to the other reservoirs.

A spring-loaded valve on the reservoir that opens and releases air if pressure rises above a set maximum. It protects tanks and lines from bursting.

A valve that allows air to flow in one direction only. It stops air from draining back out of a full tank when pressure drops elsewhere in the system.

The pedal-operated valve under the driver's foot. Pressing it meters air from the reservoirs into the brake chambers, so more pedal travel means more braking force.

A sealed can containing a rubber diaphragm and a push rod. Air pressure pushes the diaphragm, which drives the push rod outward to work the slack adjuster and brakes.

The rod that moves out of the brake chamber when air is applied. How far it travels, called stroke, shows whether the brakes are properly adjusted.

A lever on the brake camshaft that turns push rod movement into rotation of the cam. Adjusting it controls how much push rod travel is needed to apply the brakes.

An S-shaped cam on the brake camshaft. As the push rod and slack adjuster rotate it, its shape forces the brake shoes outward against the drum.

The round metal part that spins with the wheel. Brake shoes press against its inside surface, and the resulting friction slows and stops the vehicle.

The friction material attached to the brake shoes that rubs against the drum. Grease, oil, or excessive wear on the lining reduces stopping power.

A system with two separate air circuits, each with its own reservoir and lines. If one circuit fails, the other still supplies air so the vehicle can be stopped.

The air circuit that normally works the rear service brakes. Because it has its own tank and lines, a leak elsewhere does not take away its pressure.

The air circuit that normally works the front brakes. It is a backup that keeps braking available if the primary circuit loses air.

A valve mounted near the rear axles that takes a small signal from the foot valve and delivers a large, fast flow of air to the rear brake chambers.

A valve that dumps air out of the brake chambers as soon as the pedal is released, so the brakes let go faster and brake lag is reduced.

A valve that shuts off the air lines to the trailer when pressure drops too low or the lines break. The tractor keeps its air, and the trailer brakes apply.

A brake chamber containing a strong spring that applies the brakes whenever air is released. Air pressure must be supplied to release it, and it serves for parking and emergencies.

A valve that stops the spring brake and the service brake from pushing on the same chamber at the same time, preventing damage from the combined force.

A valve that lets the spring brakes be released and applied gradually with the dash control when the service brake system has failed, giving limited stopping control.

The required check before driving that includes air pressure build-up, leak tests, warning signals, slack adjuster and push rod checks, and a test of the brakes.

With the engine off, full pressure in the tanks, and the brakes released, the driver watches the gauges for one minute. Pressure loss beyond the limit means a leak.

With the engine off and the brakes held fully on, the driver watches the gauges for one minute. Excess pressure loss shows a leak in the applied side of the system.

Running the engine at fast idle and timing how long pressure takes to climb from about 85 to 100 psi. A slow build-up points to a weak compressor or a leak.

The high pressure at which the compressor stops pumping (cut-out, typically near 120 to 125 psi) and the lower pressure at which it restarts (cut-in, often about 20 psi lower).

The longest push rod stroke allowed for a given chamber size. When travel goes beyond the limit, the brakes must be adjusted or repaired before the vehicle is driven.

Pulling the slack adjuster by hand to see how far it moves before the brakes begin to apply. Too much free play means adjustment or repair is needed.

The short delay between pressing the pedal and the brakes gripping at the wheels, caused by air traveling through the lines. It adds to total stopping distance.

Squeezing on steady, firm pedal pressure and holding it to slow or stop without locking the wheels. It produces smooth stops and keeps the vehicle straight.

Loss of stopping power when drums and linings get too hot from repeated or long braking. The pedal may still feel firm even though the brakes barely work.

Applying the brakes hard, holding until the wheels are nearly stopped from rolling, then releasing and repeating. It is used when a quick stop is needed.

A light and buzzer that come on when tank pressure falls to about 60 psi. The driver must stop safely and find the cause before moving on.

When air pressure falls to roughly 20 to 45 psi, the spring brakes set by themselves and bring the vehicle to a stop. Ignoring a low pressure warning leads to this.

If the brakes fail: keep steering, downshift, pump the pedal, apply the parking brake gradually, and look for an escape route. Do not shut off the engine.

The dash control that releases air from the spring brake chambers to set the parking brakes, and supplies air to release them. It is pulled out to park.

Set the parking brake, use chocks or turn the wheels toward the curb, and leave the transmission in the lowest gear. Never depend only on air to hold a parked vehicle.

On a long downgrade, brake firmly to cut speed by 5 to 10 mph, then release and let the brakes cool before repeating. It keeps speed low without overheating.

Brakes that grab or pull weakly after driving through deep water. Applying light pedal pressure while rolling slowly heats and dries the linings until braking feels normal.