The LABS test is a method for remembering all the steps to testing the air brake system on a commercial vehicle, which should be completed during every pre-trip inspection.

Performing the air brakes LABS test

Nov. 13, 2018
Every pre-trip inspection should include the LABS test to test the air braking system on a commercial vehicle.

One critical step in the pre-trip inspection is to make sure your air brakes are in good operating order before hitting the road. As a student preparing for the pre-trip inspection portion of the CDL test, I need to know the proper procedure for conducting this test. There are four steps to take to ensure all areas of the air braking system are working properly. 

(Read more about the entire pre-trip inspection process in a previous blog post here.)

The LABS test

The LABS test is a method for remembering all the steps to testing the air brake system on a commercial vehicle, which should be done during a pre-trip inspection. LABS stands for Leaks, Alarms, Buttons, and Service Brake. It allows you to confirm the following (these tests are conducted in order):

Leaks - after the wheels are chocked, and the air pressure built up to between 90-125 psi, the driver should leave the vehicle in neutral and cut the engine. Then, turn the key to the "on" position. (Some vehicle alarms will not work unless the vehicle's auxiliary power is available.) The next step is to press in the tractor (and trailer, if applicable) parking brake to release it. After the initial pressure drop, the driver then engages the brake pedal at a steady pressure for 60 seconds. The air pressure cannot drop more than 3 psi for a tractor, and no more than 4 psi for a combination vehicle (aka, tractor with a coupled trailer).

Alarms - You then must confirm the alarms work to alert drivers of a drop in air pressure. To do this, you must apply and release pressure on the foot brake until the audible or visual alarm alerts to a drop in pressure  to no more than 60 psi.

Buttons - This test confirms the tractor (and trailer, when applicable) parking brakes are in working order. To do this, the driver must continue to apply and release pressure on the foot brake until the parking brake knob(s) pop out. This must happen before pressure drops to 20 psi, to ensure the failsafe is in proper working order.

Service brakes - Before conducting this test, the driver can grab the wheel chocks.  The last portion of the test requires the driver to build up air pressure again to 120-125 psi, in order to test the service brakes. Once pressure is built, place the vehicle in a low gear and release the clutch and brake to move forward at about 5 mph. We’re taught to hover our hands at the 10 o’clock and 2 o’clock positions while the vehicle moves forward. Once the vehicle is in motion, you should push in the clutch pedal and brake pedal. If the wheel jerks to either side when applying pressure to the brakes, it indicates that side of the vehicle may have a brake issue. If not, you can confirm the service brakes are in proper working order.

One of the instructors at WCTC taught me that every part of the LABS test should be conducted with the driver keeping his or her hands on the wheel the entire time. This ensures you do NOT touch the tractor or trailer parking brake during the test. If you do touch the buttons during the CDL pre-trip test, you will have to start the entire LABS test again from the beginning.

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In the Driver’s Seat is a blog series from Fleet Maintenance Editor-in-Chief Erica Schueller. The series covers the process of attending a formalized technical training professional truck driving program in order to obtain a Class A commercial vehicle driver’s license.

For more updates, follow her on Twitter and Instagram.

About the Author

Erica Schueller | Media Relations Manager | Navistar

Erica Schueller is the Media Relations Manager for Navistar.

Before joining Navistar, Schueller served as Editorial Director of the Endeavor Commercial Vehicle Group. The commercial vehicle group includes the following brands: American Trucker, Bulk Transporter, Fleet Maintenance, FleetOwner, Refrigerated Transporter, and Trailer/Body Builders brands.

An award-winning journalist, Schueller has reported and written about the vehicle maintenance and repair industry her entire career. She has received accolades for her reporting and editing in the commercial and automotive vehicle fields by the Truck Writers of North America (TWNA), the International Automotive Media Competition (IAMC), the Folio: Eddie & Ozzie Awards and the American Society of Business Publication Editors (ASBPE) Azbee Awards.

Schueller has received recognition among her publishing industry peers as a recipient of the 2014 Folio Top Women in Media Rising Stars award, acknowledging her accomplishments of digital content management and assistance with improving the print and digital products in the Vehicle Repair Group. She was also named one Women in Trucking’s 2018 Top Women in Transportation to Watch.

She is an active member of a number of industry groups, including the American Trucking Associations' (ATA) Technology & Maintenance Council (TMC),  the Auto Care Association's Young Auto Care Networking Group, GenNext, and Women in Trucking.

In December 2018, Schueller graduated at the top of her class from the Waukesha County Technical College's 10-week professional truck driving program, earning her Class A commercial driver's license (CDL).  

She has worked in the vehicle repair and maintenance industry since 2008.

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