Every CDL Term Explained

Air brakes are standard on most commercial trucks and many trailers. Drivers must pass an air brakes endorsement test or demonstrate air brake knowledge during their CDL test. Operating a vehicle with air brakes without the endorsement is a violation.

An air compressor builds air pressure stored in reservoirs. When you press the brake pedal, air pressure is sent to the brake chambers, which push the slack adjusters, rotate the brake cams, and press the brake shoes against the brake drums.

The air compressor is driven by the engine. It pumps air into the reservoir tanks. The air governor controls when the compressor builds pressure (cut-in) and when it stops (cut-out). Typical cut-out pressure is 120–135 PSI.

The governor sets the cut-in pressure (when the compressor starts building) and cut-out pressure (when it stops). If the governor fails, the compressor may over-pressurize the system or fail to maintain adequate pressure.

Under federal regulations, the maximum weight on a single axle is 20,000 lbs. Tandem axles (two axles close together) are limited to 34,000 lbs. Total gross vehicle weight is typically limited to 80,000 lbs. States may have different limits.

Tractor-semitrailer (18-wheeler), doubles (two trailers), triples (three trailers), and B-trains. Each has different handling characteristics and may require additional endorsements.

Level 1 is the most comprehensive — covers the driver's documents and the entire vehicle. Level 2 is a walk-around inspection without going under the vehicle. Level 3 is driver-only. Level 6 is for radioactive material. Level 7 is for transporting school children.

If a critical safety violation is found, the vehicle or driver can be placed Out of Service — meaning they cannot operate until the violation is corrected.

The fifth wheel is a U-shaped plate with a locking jaw mechanism. When the trailer is backed onto the tractor, the kingpin enters the opening and the jaw closes around it, locking the two vehicles together. The fifth wheel allows the trailer to pivot during turns.

During pre-trip inspection, drivers verify the fifth wheel is properly lubricated, the locking jaw is secure around the kingpin, no cracks in the mounting, and the safety latch is engaged.

Pre-trip inspection, CDL skills test, coupling and uncoupling procedures, and in-cab orientation at trucking companies.

GCWR determines which CDL class is required. If the GCWR of a combination exceeds 26,001 lbs., a Class A CDL is typically required. Exceeding the GCWR can damage the vehicle and create legal liability.

Glad hands are color-coded to prevent mis-connection. The blue glad hand connects to the service line. The red glad hand connects to the emergency (supply) line. Connecting them incorrectly can disable the brakes.

Drivers check glad hands for cracks, proper sealing, and secure connection during pre-trip inspection.

GVWR determines CDL class requirements. A single vehicle with a GVWR over 26,000 lbs. requires a Class B CDL. A combination vehicle with a GCWR over 26,001 lbs. requires a Class A CDL.

Using wheel brakes continuously on a long downhill grade can cause brake fade — the brakes overheat and lose effectiveness. Engine brakes help control speed without overheating the friction brakes.

Many cities restrict the use of engine/compression brakes in residential areas due to noise. Watch for posted signs.

Inspect the kingpin for bends, cracks, or wear. A damaged kingpin can fail during operation, causing the trailer to separate from the tractor.

Failing to raise the landing gear fully before moving the tractor can drag the landing gear on the road surface, causing damage and potential accidents. Always verify landing gear is fully retracted before departure.

Most modern trucks use automatic slack adjusters that self-adjust as the brake shoes wear. Manual slack adjusters require periodic adjustment. During pre-trip inspection, drivers check stroke length — if the pushrod travel is too long, the brakes may be under-adjusted and out of service limits.

Apply air pressure and check that the pushrod stroke is within the legal limit. Excessive stroke indicates the brakes need adjustment and may result in an Out of Service violation.

Sliding the tandems forward shifts more weight to the drive axles. Sliding them back shifts weight toward the trailer axles. Drivers adjust them to stay within legal axle weight limits or to comply with bridge law formulas.

Spring brakes are a fail-safe system. If air pressure is lost due to a leak or failure, the springs automatically apply the brakes. Air pressure must be built up to release (hold back) the spring before the truck can move.

Pushing in the yellow diamond-shaped valve on the dash releases air from the spring brake chambers, allowing the springs to engage and hold the vehicle parked.

The federal maximum for a steer axle is 12,000 lbs. Some states may allow higher limits with proper permits.

Federal regulations allow up to 34,000 lbs. on a set of tandem axles. This is more than twice the single-axle limit of 20,000 lbs.

Dry van (enclosed box), refrigerated (reefer), flatbed, tanker, lowboy, step-deck, curtainside, and specialized trailers for oversized or hazardous cargo.