Who this is for: CDL holders with air brake restriction, fleet managers
Air Brake Restriction (L) — How to Remove It from Your CDL
The air brake restriction (code L) is placed on a CDL when the skills test was taken in a vehicle without air brakes. To remove it, you must pass an air brakes knowledge test and demonstrate a pre-trip inspection of an air brake-equipped vehicle.
Why restriction L is applied
If you took your CDL skills test in a vehicle with automatic transmission only or no air brakes, the state places restriction L on your CDL. This prevents you from legally operating CMVs equipped with air brakes — which includes most Class A trucks.
Steps to remove restriction L
Removing the air brake restriction generally requires: (1) passing the air brakes section of the CDL knowledge test, and (2) completing an air brake vehicle inspection during a skills test appointment. Some states may require a full skills test re-administered in an air brake vehicle. Contact your state CDL office to confirm the exact process.
Why removing this restriction matters for drivers
Most over-the-road trucks use air brakes. A CDL with restriction L significantly limits employment opportunities. Fleet managers should verify whether drivers have this restriction when assigning vehicles with air brakes.
Air brake fundamentals — why they require a separate test
Air brakes work on a fundamentally different principle from hydraulic brakes found in most passenger vehicles. Hydraulic brakes fail when pressure is lost; air brakes fail safe — the spring brakes engage when air pressure drops below a threshold, stopping the vehicle. Understanding this distinction matters for how drivers respond to brake system issues. The knowledge test covers brake fade, brake lag (the delay between pedal application and braking effect), inspection procedures, adjustment requirements, and how to identify a brake system problem during a pre-trip.
What the air brakes knowledge test covers
The air brakes section of the CDL knowledge test typically addresses: air compressor operation and governor cut-in/cut-out pressures; air pressure gauge readings; low pressure warning devices; spring brakes and the parking brake; brake adjustment (slack adjusters); pre-trip inspection procedures for the air brake system; and proper use of brakes on grades. Most state CDL manuals have a dedicated air brakes chapter — study it in full, not just the summary points.
The pre-trip air brake inspection — what inspectors check
A pre-trip inspection of an air brake vehicle follows a specific sequence. The driver builds air pressure to the cutoff point, turns off the engine, and checks for pressure loss over one minute (no more than 2 PSI loss for a single vehicle, 3 PSI for a combination). The spring brakes must release at the correct pressure threshold. Low pressure warning devices are tested by drawing down pressure. Brake pedal push-down tests check for consistent pressure. These steps are observed during the skills test and during roadside inspections — skipping them is an out-of-service risk.
The manual transmission restriction (E)
The air brake restriction is the most common, but there's a companion restriction worth knowing: restriction E (no manual transmission) is placed on CDLs when the skills test was taken in an automatic transmission vehicle. This prevents the driver from operating manual-shift CMVs. To remove restriction E, the driver must retake the skills test in a vehicle with a manual transmission. Both restrictions can be removed in the same appointment if the test vehicle has both air brakes and a manual transmission.
Frequently Asked Questions
Can I remove restriction L at my CDL renewal?
Possibly — you can request to remove the restriction by completing the air brakes test at your state CDL office, which can sometimes coincide with a renewal appointment. Plan ahead since scheduling can take time and some states require a separate appointment from renewal.
Does restriction L apply to a vehicle with only front air brakes?
Generally yes — if the vehicle has any air brakes, restriction L applies unless it has been removed. Vehicles with air brakes only on certain axles are still considered air brake vehicles under CDL regulations.
What happens if a fleet assigns an air-brake vehicle to a driver with restriction L?
The driver is in violation for operating outside their CDL restrictions, and the carrier is in violation for knowingly allowing it. The driver faces an OOS order and potential CDL disqualification; the carrier faces civil penalties. Fleet managers should verify restriction codes during onboarding and at each annual MVR review — restriction L is visible on the MVR and the CDL itself.
How difficult is the air brakes knowledge test?
The test typically has 25 questions drawn from the air brakes chapter of the state CDL manual. Topics include compressor function, air pressure thresholds, pre-trip inspection procedures, brake adjustment, and failure modes. Most drivers who study the manual chapter directly pass on the first attempt. The knowledge test can be taken at the DMV without a vehicle and does not require scheduling an appointment with a test vehicle.