NC CDL Air Brakes flashcards

31 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 125 psi; above this point the safety valve protects the system from overpressure.

Governor cut-in pressure

The pressure at which the governor restarts the compressor, typically about 100 psi; pressure cycling between cut-in and cut-out is normal system operation.

Low air pressure warning

A red light and/or buzzer that must activate before system pressure drops below 60 psi, warning the driver that spring brakes will soon apply automatically.

Spring brake chamber

A brake chamber containing a strong mechanical spring held off by air pressure; when pressure is lost, the spring applies the brake automatically as an emergency and parking brake.

Relay valve function

A valve that uses a small control signal from the foot valve to supply reservoir air quickly to the brake chambers, speeding application and release on long vehicles.

Supply (wet) tank

The first tank after the compressor; it collects moisture and oil, has a drain cock, and should be drained daily to prevent water from reaching valves and chambers.

Safety valve setting

A pressure relief valve on the supply tank, usually set near 150 psi, that opens to release excess air if the governor fails and pressure climbs too high.

Air dryer purpose

A device downstream of the compressor that removes moisture and oil vapor from the air before it reaches the tanks, preventing freeze-ups and corrosion in the brake system.

Leak test procedure

With the engine off and system charged, release the parking brake and watch the gauge; a drop over 2 psi per minute (single) or 3 psi (combination) indicates a leak.

Low pressure warning test

With the engine off and key on, fan the brakes to lower pressure; the warning light and buzzer must come on before the gauge falls below 60 psi.

Spring brake test

With the system charged, shut off the engine and fan pressure down; the parking brake should apply automatically when pressure reaches the 20-45 psi range.

Service brake test

With the engine running and pressure built, hold the pedal down and check the gauge; a loss of more than 3 psi per minute indicates a leak in the service system.

Compressor build rate

With the engine at operating rpm, pressure should rise from 85 to 100 psi within 45 seconds for a single vehicle or 3 minutes for a combination vehicle.

Slack adjuster check

With brakes released, mark the pushrod, apply the brakes fully, and measure the stroke; a stroke beyond the manufacturer's limit means the brake is out of adjustment.

Tank drain procedure

Open the drain cocks on all air tanks daily and let moisture and oil escape until only clean air comes out, then close them to prevent freeze-ups and corrosion.

Air compressor belt check

Inspect the compressor drive belt for cracks, glazing, and correct tension; a slipping or broken belt causes slow pressure build-up and possible brake failure.

Three braking systems

Air brakes combine the service brake (foot valve, modulated), the parking brake (spring applied, air released), and the emergency brake (spring brakes applied by pressure loss).

Brake fade cause

Heat from continuous braking reduces friction between drum and lining, weakening braking force; control speed by gearing down and using intermittent, firm brake applications.

Downgrade braking technique

Select a low gear so engine braking holds speed, apply the service brakes firmly enough to feel them work, then release to let them cool; never ride the brakes.

Air brake lag time

A delay of about 0.2 to 0.4 seconds between pressing the pedal and brake application, which increases stopping distance compared with hydraulic brakes.

Parking brake control

The yellow diamond knob; push in to release the spring brakes and pull out to apply them. It should not be used as a service brake at speed.

Treadle (foot) valve

The driver's service brake control; it meters air pressure to the relay valve in proportion to pedal travel rather than sending fluid to the wheels.

Combination brake balance

Trailer brakes should apply slightly before or with the tractor brakes; if the trailer pushes the tractor, check the relay valve and brake adjustment.

Emergency braking on grade

If brakes fail on a downgrade, downshift, pull the parking control to apply the spring brakes, and steer toward a runaway ramp or escape route.

Low pressure while driving

If the warning sounds, stop as soon as it is safe; the spring brakes will apply automatically when pressure falls to the 20-45 psi range.

Rapid pressure loss

A broken line or failed chamber causes fast pressure loss; stop immediately, because the spring brakes will lock the wheels and steering control may be affected.

Dragging brake symptom

A smoking wheel or hot drum can result from a stuck slack adjuster or a failed spring brake chamber; stop and let it cool before continuing.

Frozen brake line

Moisture in the system can freeze and block air flow in cold weather; prevent it with air dryer maintenance and alcohol evaporator use, not by pouring hot water on lines.

Stuck parking brake valve

If the parking brake will not release because of a stuck valve, the vehicle cannot be moved safely; call for service rather than forcing the control.

Spring brake reapplication

After automatic spring brake application, release the parking control only after pressure rebuilds above the cut-in point; otherwise the brakes will reapply.

Compressor cannot keep up

A continuous leak from a failed chamber or hose may exceed compressor output; the low pressure warning sounds and the spring brakes engage, forcing a stop.