FL CDL Air Brakes flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Governor cut-out pressure

The pressure at which the governor stops the compressor from pumping, typically about 120-125 psi. Above this, the safety valve may vent.

Wet tank

The first reservoir to receive air from the compressor. It collects moisture and oil, and it is the tank drained most often.

Type 30 brake chamber

A chamber with 30 square inches of effective piston area. At 100 psi it produces about 3,000 pounds of force on the pushrod.

Slack adjuster

The lever that links the chamber pushrod to the S-cam. Its angle should be about 90 degrees at full brake application.

Relay valve

A valve controlled by the foot valve that supplies a large volume of air quickly to the rear brake chambers, speeding application and release.

Quick release valve

A valve at each axle that exhausts air from the chambers quickly when the pedal is released. A failure causes slow release and dragging brakes.

Air dryer

A device downstream of the compressor that removes moisture and oil vapor. A failed dryer sends water into tanks and valves.

Dual air brake system

A system with two separate circuits, primary and secondary, each with its own reservoir. One circuit can fail and the other still stops the vehicle.

Check valve

A one-way valve that prevents air from flowing back from a charged tank to a depleted one, keeping the two circuits independent.

Spring brake

A parking brake applied by a heavy spring when air is released, and released only when pressure builds above roughly 45-60 psi.

Modulator valve

A valve that prevents the spring brakes from applying during a service brake application, and applies them if service pressure is lost.

Tractor protection valve

A valve that closes the trailer supply line at about 45 psi and the trailer service line at about 20 psi, protecting the tractor if the trailer breaks away.

Front-brake limiting valve

A valve that reduces front brake pressure in the bobtail condition, lowering the risk of front wheel lockup on an unloaded tractor.

Static leak test

With the engine off and parking brake released, hold the foot valve applied for one minute. The pressure drop must be no more than 3 psi.

Compressor build test

With the engine at fast idle, the system should build from 85 to 100 psi within 45 seconds. Slower build indicates a compressor or leak problem.

Low-pressure warning check

With the engine off and parking brake released, the warning light and buzzer must activate before pressure falls below 60 psi.

Spring brake apply check

With the engine off and parking brake released, the spring brakes must fully apply by about 45 psi, and the vehicle must not roll.

Pushrod stroke limit

The maximum pushrod travel allowed for a chamber, typically 1 inch for a type 30 chamber at 90 psi. Over the limit is an out-of-service defect.

Normal operating pressure

The dash gauge should read roughly 100-125 psi while driving. Pressure that keeps falling signals a leak in the system.

Safety valve

A valve on the wet tank that relieves pressure at about 150 psi, protecting the system if the governor fails to cut out.

Daily tank drain

Draining the wet and dry tanks each day. Water or oil in the drain indicates a failed air dryer or a worn compressor.

Low-pressure warning

A light and buzzer that must activate before pressure falls below 60 psi, warning the driver that braking capacity is being lost.

Air dryer purge cycle

The dryer should purge about every 30 seconds at highway speed. Frequent purging indicates a leak or a worn compressor.

Low-pressure response

If the warning sounds while driving, stop as soon as it is safe, set the parking brake, and do not continue until the leak is found.

Compressor failure

The reservoirs hold enough air for only a limited number of brake applications. Use them sparingly and stop the vehicle early.

Trailer breakaway

If the trailer separates, the tractor protection valve closes the lines and the trailer spring brakes apply automatically, stopping the trailer.

Spring brake manual release

A mechanical release on the chamber that lets you move a vehicle whose spring brakes have applied and cannot be released with air.

Brake lag distance

The distance traveled during the roughly 0.4-0.5 seconds before air pressure reaches the chambers, about 32-40 feet at 55 mph.

Total stopping distance

The sum of perception distance, reaction distance, brake lag distance, and effective braking distance. Each part grows with speed.

Threshold braking

Applying firm, steady pressure just short of lockup. Do not pump the pedal on a vehicle equipped with ABS.

ABS braking technique

Hold firm, continuous pressure and steer. The ABS cycles the brakes and the pedal may pulse; do not pump the brake.

Brake fade

Loss of stopping power from overheated drums or discs. Stop and let the brakes cool rather than pressing the pedal harder.