NE 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 unloads the compressor, commonly about 120 to 125 psi, stopping further pressure buildup until the tank pressure falls to the cut-in point.

Governor cut-in pressure

The lower pressure at which the governor lets the compressor load and build air again, typically about 20 psi below cut-out, keeping the system within its operating range.

Air dryer function

A device between compressor and wet tank that removes moisture and oil vapor from compressed air, preventing water from reaching valves and brake chambers where it causes corrosion and freeze-ups.

Wet tank purpose

The first tank receiving compressor air, where most moisture and oil condense and settle; it supplies the other circuits through check valves that prevent backflow between them.

Safety valve setting

A pressure relief valve on the supply tank that opens at about 150 psi to protect the system if the governor fails and the compressor keeps pumping air.

Relay valve role

A high-volume valve controlled by a small signal from the foot valve that quickly sends tank air to the brake chambers, speeding application and release on long vehicles.

Slack adjuster function

A lever that converts pushrod travel from the brake chamber into rotation of the S-cam, spreading the shoes and multiplying chamber force into braking torque at the drum.

Spring brake chamber

A brake chamber containing a strong coil spring that applies the brake when air is released and is compressed out of the way when the chamber is charged with air.

Low-pressure warning switch

A sensor that triggers a buzzer and dash light when system pressure drops below about 60 psi, alerting the driver before braking ability is lost.

Single vehicle leak test

With the engine off and system charged, release parking brakes and hold the service brake; pressure loss must not exceed 3 psi in one minute.

Combination leak limits

For tractor-trailer combinations, allowable loss is 4 psi in one minute with brakes released and 6 psi in one minute with the service brake applied.

Spring brake test

With the engine off, fan off air and watch when the parking brake applies; it should set fully as pressure falls to roughly 20 to 45 psi.

Compressor build rate

With the engine at operating rpm, pressure should rise from about 85 to 100 psi within 45 seconds on a single vehicle, showing the compressor is healthy.

Pushrod stroke check

Measure how far the pushrod extends from the chamber when the brake is applied; a stroke at or beyond the vehicle's adjustment limit requires adjustment or repair.

Slack adjuster free play

With the brake released and pushrod fully returned, the slack adjuster should move no more than about 1/4 inch before it starts to actuate the brake.

Daily tank draining

Draining all air tanks each day removes condensed water and oil; heavy accumulation between drains indicates a failing air dryer or excessive oil from the compressor.

Automatic slack adjusters

Self-adjusting levers that maintain pushrod stroke in service; they should not be manually readjusted to hide excessive stroke, which signals a worn or failing component.

Supply-side air leak

A leak that lowers the gauge even when the brakes are not applied, such as a leaking tank, fitting, hose, or a compressor unloader that will not seat.

Service-side air leak

A leak that shows only while the brake pedal is held, commonly at a chamber diaphragm, hose, or valve in the application circuit.

Slow pressure buildup

Pressure that will not reach normal or climbs very slowly, caused by a worn compressor, slipping drive belt, stuck governor, or a major open leak.

Overpressure venting

Air escaping at the safety valve above normal cut-out means the governor or compressor unloader failed, and the vehicle must be removed from service.

Brake pull to one side

A symptom of a dragging shoe, oil or grease on a lining, a collapsed hose, or a stuck valve, causing unequal braking between wheels.

Ruptured chamber diaphragm

A torn diaphragm or cut hose prevents the chamber from holding air, causing continuous air loss and loss of braking at that wheel.

Warning stays on at pressure

If the low-pressure buzzer and light remain on while the gauge reads normal, a faulty switch or gauge is suspected and the defect must be verified before driving.

Water after draining

Significant water in the tanks right after draining shows the air dryer or alcohol evaporator is not working, since a functioning dryer removes moisture before it condenses.

Spring brake application point

Parking and emergency spring brakes apply automatically when system pressure falls to roughly 20 to 45 psi, because air no longer holds the spring compressed.

Dual-circuit protection

Separate primary and secondary circuits ensure that a failure in one still leaves braking in the other, though stopping distance lengthens and a warning appears.

Tractor protection valve

A valve that closes to stop air loss from the tractor when a trailer line separates or breaks away, preserving tractor brakes while trailer spring brakes apply.

Trailer hand valve limit

The trolley valve applies only trailer brakes and is used for testing; it must never be used for parking because it does not hold the tractor.

Downgrade gear selection

Choose a gear low enough that engine braking and any retarder hold the speed, so service brakes are used only briefly rather than continuously.

Brake fade cause

Overheated drums and linings from riding the brakes lose friction and stopping power; the remedy is engine braking and stopping to let the drums cool.

Stab braking method

Apply the brakes firmly until speed drops about 5 mph below the safe downgrade speed, release fully, and repeat when speed builds again.