CDL Air Brake Inspection

Cards · 50

Pre-start air pressure setup

Reduce system air pressure to 90 PSI or below if it is currently above 100 PSI before starting engine.

Engine start oil pressure threshold

Oil pressure must begin rising within 3 to 5 seconds after starting the engine.

Air compressor governor cut-out PSI

Normal cut-out occurs between 120 and 140 PSI when the needle stops rising.

Air compressor governor cut-in PSI

Compressor cuts in when pressure drops to approximately 100 PSI (or standard manufacturer minimum).

Governor cut-out verbal verification

State: 'My governor has cut out at [PSI]; I know this because my needle has stopped rising.'

Applied brakes test setup

Engine off, key turned to ON, parking brakes pushed in, and service brake held firmly for 1 minute.

Applied test max leakage (combination)

Maximum allowable pressure drop is 4 PSI in 1 minute for a tractor-trailer combination.

Applied test max leakage (straight truck)

Maximum allowable pressure drop is 3 PSI in 1 minute for a single vehicle.

Static leakage test max rate (combination)

With engine off and brakes released without pedal application, max loss is 3 PSI in 1 minute.

Static leakage test max rate (straight truck)

With engine off and brakes released without pedal application, max loss is 2 PSI in 1 minute.

Low air warning test activation threshold

Low air warning light and buzzer must activate at or above 55 PSI (typically 55 to 75 PSI).

Spring brake pop-out pressure range

Emergency/parking brake valves must automatically pop out between 20 and 45 PSI.

Air pressure build-up rate (dual systems)

Pressure must build from 85 to 100 PSI within 45 seconds at normal operating RPM.

Trailer parking brake tug test

Keep trailer brake set, release tractor brake, place in low gear, and gently tug against the brake.

Tractor parking brake tug test

Keep tractor brake set, release trailer brake, place in low gear, and gently pull to verify hold.

Service brake moving stop test

Drive forward at 5 mph, press service brake firmly; check for unusual feel, delay, or pulling.

Air tank drain valve function

Purges accumulated moisture and compressor oil from air tanks to prevent brake line freeze and corrosion.

Manual air tank draining schedule

Tanks equipped with manual drain valves must be drained completely at the end of every operating day.

Safety relief valve purpose

Protects the air supply system from overpressure; automatically opens around 150 PSI.

Alcohol evaporator purpose

Introduces alcohol vapor into air lines to reduce the risk of ice formation in cold weather.

Slack adjuster maximum pushrod stroke

Should generally not move more than about 1 inch from pushrod travel when pulled firmly by hand.

Brake drum friction surface inspection

Must be free of cracks longer than half the friction area, oil, and excessive wear or grooving.

Brake lining minimum thickness

Brake linings must not be dangerously thin; typically at least 1/4 inch thick at the center.

Supply pressure gauge function

Informs the driver of the exact air pressure available in the primary and secondary supply tanks.

Front brake limiting valve function

Reduces braking force on front wheels on older vehicles to lessen skids on slippery surfaces.

S-cam brake operation principle

Turning S-cam forces the brake shoes outwards against the inner surface of the brake drum.

Wedge brakes operation principle

Pushrod forces a wedge directly between brake shoes, expanding them against the drum.

Air disc brake mechanism

Chamber pushrod turns a power screw or cam, squeezing pads against both sides of a brake rotor.

Dual air brake system primary circuit

Typically operates the regular service brakes on the rear axle or drive axles.

Dual air brake system secondary circuit

Typically operates the regular service brakes on the front steering axle.

Tractor protection valve function

Closes automatically to retain tractor air if the trailer breaks away or leaks severely.

Trailer air supply control (red knob)

Pushed in to supply air to trailer tanks; pulled out to set emergency trailer brakes.

Parking brake control (yellow knob)

Diamond-shaped valve pushed in to release all parking brakes; pulled out to apply them.

Service air line (blue line)

Carries air commanded by the foot brake or hand valve to apply trailer service brakes.

Emergency air line (red line)

Supplies constant air to trailer air tanks and controls the trailer emergency spring brakes.

Glad hand rubber seal inspection

Seals must be clean, pliable, free of cracks, and seated properly to prevent air leaks.

Dead-end dummy couplers purpose

Protects disconnected glad hands from dirt and water contamination when not towing a trailer.

Trailer hand valve (trolley valve) rule

Used only to test trailer service brakes; never use for parking or to prevent rollaways.

Brake lag distance definition

Distance traveled between pressing the brake pedal and air pressure mechanically engaging brakes.

Typical air brake lag time

Approximately 0.5 seconds or more for air to flow through valves to brake chambers.

Total stopping distance components

Perception distance plus reaction distance plus brake lag distance plus braking distance.

Brake fade cause

Excessive heat causing brake drums to expand away from linings and friction material to glaze.

Proper braking technique on steep downgrades

Select low gear, brake to 5 mph below safe speed, release, and repeat (snub braking).

Spring brakes held off mechanism

Continuous pneumatic pressure compresses a heavy mechanical spring to release the brake.

Modulating control valve purpose

Allows gradual application of spring brakes via a lever if primary air systems fail.

ABS malfunction indicator lamp on startup

Yellow ABS lamp turns on during bulb check and turns off shortly after if functional.

Effect of ABS failure on braking

Normal mechanical air braking remains fully functional even when ABS electronics fail.

Air compressor air filter inspection

Must be regularly inspected and cleaned to prevent dirt entering compressor cylinders.

Chocking wheels during brake tests

Place wheel chocks before releasing parking brakes on any grade during brake tests.

Danger of overriding spring brakes

Never push brake pedal down when spring brakes are applied; compounding can damage components.