Instructor
CDL Study Buddy Team
The air brake test is a mandatory component of the CDL skills exam for any vehicle equipped with air brakes. This video provides a complete step-by-step demonstration of the three-part air brake test that examiners require, ensuring you understand each procedure and can perform it correctly.
The first part of the test — the air pressure build-up check — verifies that the air compressor can build pressure from 50 to 90 PSI within the required time limit of three minutes. Our instructor demonstrates how to start the engine at fast idle and monitor the primary and secondary air pressure gauges until they reach the target pressure. You will learn what acceptable build-up rates look like and how to identify a compressor or system leak problem.
The second part — the governor cut-out and cut-in test — demonstrates how to verify that the air compressor governor properly regulates system pressure. The instructor explains that the governor should cut out (stop compressing) at approximately 120-130 PSI and cut back in (resume compressing) at approximately 100-110 PSI. You will see how to repeatedly fan the service brake to reduce pressure and observe the governor cycling.
The third part — the air loss rate test — is perhaps the most critical. This test measures how much air pressure the system loses over one minute with the engine off and the brakes released, and then with the service brake fully applied. The instructor explains the maximum allowable leak rates: no more than 2 PSI per minute for a single vehicle and 3 PSI per minute for a combination vehicle with the brakes released, and 3 PSI and 4 PSI respectively with the brakes applied.
You will also learn the importance of the low air pressure warning light and buzzer, which must activate before pressure drops below 60 PSI. The emergency spring brake activation test is demonstrated — showing how the parking brake valve (yellow diamond knob) should automatically pop out when pressure drops to approximately 20-45 PSI, indicating the spring brakes are engaging.
Additionally, the video covers the tractor protection valve test for combination vehicles, verifying that the trailer air supply valve (red octagonal knob) functions correctly and that the trailer emergency brakes activate when system pressure drops.
Understanding air brakes is essential not just for passing your test but for safe operation of any air-braked commercial vehicle. For the official FMCSA air brake regulations, visit https://www.fmcsa.dot.gov/. Pair this video with our free CDL Air Brakes practice test for complete preparation.
