ME 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 building more air, typically around 120 to 125 psi on most heavy-duty trucks.

Governor cut-in pressure

The pressure at which the governor lets the compressor resume building air after cut-out, typically around 100 psi.

Relay valve function

A valve that uses a small control signal to supply reservoir air directly to the rear brake chambers, speeding application and release.

Spring brake chamber

A brake chamber containing a strong coil spring that applies the brakes when air pressure is released, providing parking and emergency braking.

Slack adjuster role

A lever that converts brake chamber pushrod stroke into rotation of the S-cam, which forces the brake shoes against the drum.

Dual-circuit foot valve

A treadle valve with two separate air circuits so that one circuit can still apply brakes if the other circuit fails.

Static leak test limit

With the engine off and full pressure, loss of more than 2 psi per minute for a single vehicle or 3 psi for a combination indicates a leak.

Applied leak test limit

With the foot valve fully applied and the engine off, a drop of more than 3 psi per minute (single) or 4 psi (combination) indicates a leak.

Low-pressure warning check

With the engine off, pump the brake and confirm the warning buzzer or light activates before pressure falls below 60 psi.

Spring brake application test

With the parking brake released and the engine off, pump the brake until the spring brakes apply automatically, typically between 20 and 45 psi.

Pushrod stroke check

Mark the pushrod, fully apply the brakes, and measure travel; a stroke at or beyond the manufacturer's limit means the brakes are out of adjustment.

Tug test procedure

With the spring brakes applied and the engine at low idle in a low gear, gently release the clutch; the vehicle must not move.

Air brake lag

The delay while air pressure travels through lines before brakes apply, which makes stopping distance longer than with hydraulic brakes.

Brake fade cause

Overheated drums and linings lose friction and stopping power, often from continuous heavy braking on a long downgrade.

Downgrade braking technique

Select a gear that holds the desired speed, apply the brakes firmly only long enough to slow below the safe speed, then release.

Trailer supply valve

The red diamond valve that charges the trailer air system when pushed in; pulling it out applies the trailer spring brakes.

Wet brake behavior

Wet linings may grab or pull to one side; applying light brake pressure while moving slowly helps dry them.

Low-pressure response

If the warning activates while driving, stop as soon as it is safe, because continued driving risks automatic spring brake application.

Caging a spring brake

A mechanical bolt that compresses and holds the spring so the brake releases without air, used to move a disabled vehicle.

Compressor failure sign

Air pressure will not build or falls steadily; stop driving because reserve air is limited and repeated applications will deplete it.

Total brake failure action

Downshift, use engine braking, and look for an escape ramp or safe runoff; pumping the brake will not restore a failed air system.

Tractor protection valve

A valve that closes when system pressure is lost, retaining tractor air and causing the trailer spring brakes to apply.

Manual slack adjustment

With brakes released and normal pressure, turn the adjusting nut until linings contact the drum, then back off the specified amount.

Over-stroked automatic slack

A pushrod stroke beyond the limit on an automatic slack adjuster indicates a mechanical problem, not a need for manual adjustment.

Reservoir draining

Regularly draining air tanks removes water and oil; in cold weather, trapped moisture can freeze and block lines or valves.

Air dryer inspection

Check for saturated desiccant or a failed purge valve; moisture carryover damages valves and can cause freeze-ups in cold weather.

ABS warning lamp

A lamp that stays on after the vehicle reaches about 5 mph indicates an ABS fault; normal braking remains but lockup protection may be lost.

Engine brake caution

A compression-release retarder that slows the vehicle but can cause wheel slip on slippery surfaces and is prohibited in some areas.

Trailer hand valve

A valve that applies only the trailer brakes; it must never be used for parking and can cause a jackknife if used while moving.

Air brake endorsement

A CDL authorization, obtained by passing the air brake knowledge test and inspection, required to operate vehicles equipped with air brakes.

Out-of-service brake defect

A defect such as a pushrod stroke beyond the limit or a leaking chamber that requires the vehicle to be removed from service until repaired.

Safety valve setting

A valve on the supply reservoir that releases near 150 psi to protect the system from overpressure; one that never opens is defective.