Home / Driving & DMV Tests / Air Brakes
CDL Class A · 8 Essential Terms
CDL air-brake endorsement themes from public manuals: compressor and governor, glad hands, slack adjusters, spring brakes, the low-air warning, brake lag, and ABS. Use your state manual's exact psi figures.
License law is state-specific and manuals are revised. Confirm every item against the candidate handbook for the state where you will test.
Study desk
The pump, driven by the engine, that compresses air into the storage tanks so the brake system has a supply.
Flip cards, the timed match, and the quiz open in this panel. The full glossary below is ready either way.
Every term, pronunciation, definition, memory hook, and exam trap is written out here so you can review the whole set or print it.
air kum-PRES-er
The pump, driven by the engine, that compresses air into the storage tanks so the brake system has a supply.
Mnemonic. The compressor fills the tanks. It does not itself apply the brakes.
Exam trap. A healthy compressor that never builds pressure still fails the test if the leak is downstream. The exam wants you to know where the air is supposed to go, not to assume the pump is the only part.
GUV-er-ner
The device that tells the compressor when to pump air into the tanks and when to unload. Manuals often teach a cut-out near 125 psi.
Mnemonic. The governor is the compressor's on-off decision, based on tank pressure.
Exam trap. Memorize the figure in the manual you were issued. A question that says the compressor never stops pumping is describing a governor that will not cut out.
glad handz
The couplers that connect the tractor's service and emergency air lines to the trailer. The rubber seals wear and are a common leak point.
Mnemonic. Glad hands are the handshake between the tractor's air and the trailer's air.
Exam trap. Crossing the lines, or coupling them without a seal, is the trap. Emergency and service are not interchangeable hoses.
slak uh-JUS-ter
A lever in the foundation brake that connects the pushrod to the brake cam or shaft. Too much play means the stroke is out of limit and the brake is out of adjustment.
Mnemonic. The slack adjuster takes up slack. Excess travel means the brake is not really there.
Exam trap. The trap is a driver who never looks at stroke because the pedal feels normal. Air brakes can feel fine and still be past the adjustment limit.
spring brayk
A brake held off by air pressure and applied by a powerful spring when air pressure is lost or when the parking brake is set. It is the parking and emergency brake on most air-braked rigs.
Mnemonic. Air holds the spring back. Lose the air, and the spring does the braking.
Exam trap. The spring brake is not the regular service brake. Riding with the parking brake partially applied overheats and destroys the system. Manuals often teach automatic application somewhere in the 20 to 45 psi range.
loh air WAR-ning
The light and buzzer that must come on before tank pressure falls below the manual's threshold, commonly taught as before 60 psi, so the driver still has time to stop.
Mnemonic. The warning is an early alarm, not the last pound of air.
Exam trap. If the buzzer waits until the spring brakes have already applied, it has failed the usual test standard. Confirm the exact psi in your manual and do not round it into a guess.
brayk lag
The delay between the moment the driver presses the pedal and the moment the brakes work at the wheels, because air takes time to flow. The trailer adds more delay.
Mnemonic. Lag is the wait between your foot and the wheels. It gets longer as the air has farther to travel.
Exam trap. The trap is braking a combination rig with passenger-car timing. The extra half-second-or-more that manuals describe is why the following gap has to be larger.
ay bee es
An antilock system that releases and reapplies brake pressure to keep wheels from locking. If it is working, you brake normally. You do not pump the pedal.
Mnemonic. ABS wants one firm application. Pumping fights the system.
Exam trap. An amber ABS lamp is a fault in the antilock function, not proof that the vehicle has no brakes. The trap is refusing to brake normally because the lamp is on, or pumping because that is what an old slippery-road habit taught.