Instructor
CDL Study Buddy Team
The in-cab air brake check is performed before you leave the driver's seat and is a critical part of your CDL pre-trip inspection. This video focuses exclusively on the checks you conduct from inside the cab, ensuring you can perform each test confidently and correctly.
The video begins with the initial gauge check — key on, engine off. You will see the instructor verify that the warning light and buzzer are functioning and note the current air pressure readings. Understanding your starting pressure is essential for the subsequent tests, and the instructor explains what normal and abnormal gauge patterns look like.
The parking brake test is demonstrated first. With the engine running and the vehicle in gear (or drive for automatics), you apply light throttle against the parking brake. The vehicle should not move, confirming that the parking/spring brakes are holding properly. The instructor explains proper throttle technique and safety precautions for performing this test.
The service brake test follows, demonstrating how to check that the foot brake pedal holds firm pressure without sinking to the floor. You will learn to listen for air leaks while holding the pedal down and to watch for any warning lights that might indicate ABS or brake system malfunctions. The instructor explains the difference between a normal pedal feel and one that indicates air in the hydraulic system or a brake fluid leak.
The air pressure build-up test from the in-cab perspective is then demonstrated. You will learn to read the primary and secondary air pressure gauges simultaneously — these are the two needles on the air pressure gauge — and verify that both systems build pressure evenly. The instructor explains what it means if one needle lags significantly behind the other.
The governor cut-out test is demonstrated with the dashboard perspective, showing you how to identify cut-out by both sound and gauge reading. The instructor fans the brake pedal to reduce pressure and verifies the governor cut-in point. You will learn the acceptable pressure ranges and what to say to the examiner during this test.
The low air pressure warning system check is critical — the instructor demonstrates reducing air pressure (by repeatedly applying and releasing the service brake) until the warning light and buzzer activate. Per FMCSA requirements, this must occur before pressure drops below 60 PSI. The emergency spring brake activation is then observed as pressure continues to drop, with the parking brake valve automatically popping out.
Complete your air brake preparation with our free CDL Air Brakes practice tests and review the FMCSA brake regulations at https://www.fmcsa.dot.gov/.
