FL CDL Air Brakes key terms

An engine-driven pump that draws in outside air and compresses it to build the pressure stored in the vehicle's air tanks. It cycles on and off as the governor signals demand, and it is the source of all braking pressure.

A valve on or near the compressor that senses tank pressure and switches the compressor between building and unloading. It typically cuts the compressor in near 100 psi and unloads it near 120 to 125 psi, setting the system's operating range.

A device between the compressor and the first tank that removes moisture and oil from compressed air. Trapped water can freeze in lines or corrode valves, so the dryer and its cartridge must be serviced on schedule.

Storage cylinders that hold compressed air for braking and accessories. A dual system uses separate tanks so one circuit can still supply pressure if the other fails. Tanks collect condensate that must be drained regularly.

A petcock or cable-operated valve at the bottom of each air tank used to blow out accumulated water and oil. Draining tanks daily, or at the end of each shift, prevents moisture from reaching valves and freezing in cold weather.

An optional device that injects alcohol vapor into the air system to lower the freezing point of any moisture present. It supplements, but never replaces, proper air drying and daily tank draining.

A spring-loaded relief valve on the air tank that opens automatically if pressure climbs too high, protecting the system from over-pressurization. It is usually set to release around 150 psi and should never be plugged or adjusted.

The treadle-operated valve the driver presses to apply the brakes. It meters air from the tanks to the brake chambers in proportion to pedal travel, so pressing harder sends more pressure and produces a stronger stop.

The pedal surface of the foot valve that the driver pushes. Its free play and feel matter: a spongy or excessively loose treadle can signal air loss, a bad valve, or an improperly adjusted linkage.

A cylinder where air pressure pushes a diaphragm and pushrod to apply the foundation brake. A spring brake chamber adds a powerful mechanical spring that applies the brake automatically when air pressure is lost.

The rod that transfers force from the brake chamber diaphragm to the slack adjuster. Its travel length is a key inspection measurement: too much travel means the brakes are out of adjustment and stopping power is reduced.

The lever that connects the pushrod to the brake camshaft and converts pushrod stroke into brake application. Its angle and free play are checked during inspection because a loose or worn adjuster can leave brakes badly out of adjustment.

Setting the brakes so pushrod travel stays within the manufacturer's limit, keeping full braking force available. Automatic slack adjusters usually handle this, but drivers must still check stroke during pre-trip inspection and after any hard use.

The mechanical parts at each wheel that actually create friction and stop the vehicle, including shoes, drums or discs, and the cam or wedge that spreads them. Air pressure only actuates them; worn lining or drums reduce stopping power no matter how good the air system is.

The rotating metal cylinder that brake shoes press against to slow the wheel. Cracks, deep scoring, or heat discoloration are defects, and a cracked drum puts the vehicle out of service.

The friction material bonded to the brake shoes that contacts the drum or disc. Linings must be thick enough and free of oil or grease; linings worn thin or contaminated by hub oil are a common out-of-service condition.

A braking system with two independent air circuits, typically one for the front axle and one for the rear, each with its own tank and check valve. If one circuit loses pressure, the other still provides braking, though stopping distance increases.

A one-way valve that lets air flow into a tank but prevents it from flowing back out or between circuits. In a dual system, check valves keep a failure in one circuit from draining the other, preserving emergency braking ability.

A valve mounted near the rear brakes that uses a small control signal from the foot valve to send a large volume of air to the rear chambers. This speeds up application and release on long wheelbases and improves brake response.

A valve that vents air from a brake chamber close to the wheel instead of routing it all the way back through the foot valve. It shortens release time so brakes let go promptly when the pedal is lifted.

Emergency and parking brakes held off by air pressure and applied by heavy springs when pressure drops or the parking control is set. They apply automatically at low pressure, which is why a vehicle cannot be moved with the system bled down.

The dash control, marked with a yellow diamond, that releases air to apply the spring brakes for parking or emergency use. Pulling it applies the brakes; pushing it in recharges and releases them once pressure is sufficient.

A valve that closes automatically when trailer air pressure is lost or the trailer supply line separates, sealing the tractor's air so it keeps its own brakes. It also applies the trailer spring brakes during a breakaway.

The air line that charges the trailer's tanks and holds off its spring brakes. If it separates, pressure drops and the tractor protection valve and trailer emergency system apply the trailer brakes automatically.

The automatic application of a trailer's spring brakes if it separates from the tractor. Because the trailer loses its air supply, the springs clamp the brakes on, stopping the runaway trailer without the driver's input.

A visible and audible alert, usually a red light and buzzer, that activates before pressure falls too far, typically by 60 psi. Seeing or hearing it means stopping safely and finding the leak before moving on.

The dash gauge showing pressure in the brake system, often with separate needles for each circuit. Drivers watch it during buildup and while driving; pressure that will not build or steadily falls signals a problem.

A check of how fast pressure drops with the engine off and brakes released, then applied. A leak rate above about 3 psi per minute with brakes released, or 4 psi applied, indicates a problem needing repair.

A stationary check for leaks made with the engine off and system charged, listening for hissing and watching the gauge. It is part of the pre-trip inspection and should be done before the vehicle is driven.

The required walk-around and in-cab check of the brake system before driving, covering leaks, tank draining, warning devices, buildup rate, and brake adjustment. Finding defects on the ground is far safer than finding them in traffic.

A test in which the driver has someone observe pushrod travel while the brake is fully applied, confirming the stroke is within the adjustment limit. Excess stroke means the brakes are out of adjustment and stopping power is reduced.

A brake condition serious enough to prohibit operation until repaired, such as a cracked drum, missing lining, or stroke beyond the limit. Inspectors place vehicles out of service for these defects because braking is unsafe.

The automatic or manual application of spring brakes when air pressure is lost or the parking control is pulled. It provides stopping ability but at reduced performance, so the driver should slow and stop in a safe place as soon as possible.

The total distance a vehicle travels from the moment a hazard is seen until it stops, combining perception, reaction, and braking distance. It grows with speed, weight, and poor brake condition, and air brakes add a short lag before the brakes apply.

The brief delay between pressing the pedal and the brakes actually applying while air travels through the lines, roughly a fraction of a second. It is why drivers of air-braked vehicles must plan ahead and leave extra following distance.

The practice of braking smoothly and early, avoiding hard repeated applications that overheat brakes, and using engine braking on long downgrades. Proper technique preserves brake capacity and keeps the vehicle controllable.

The loss of braking power caused by overheating from repeated or prolonged hard braking, common on long downgrades. Faded brakes may feel normal but stop poorly, so drivers should slow down and let them cool.

Managing a slide by releasing brake pressure to let the wheels roll again and steering where you want to go. Locked wheels lose steering and traction, so the goal is to regain rolling contact rather than to brake harder.

A condition in which a wheel stops rotating and slides, usually from too much brake pressure on a slippery surface. It destroys steering control and can flat-spot tires, so the driver should ease off the pedal to restore rotation.