Inspecting and Testing Air Brakes
The pre-trip air brake test is a sequence, and exam questions often ask what a specific reading means or in what order the steps go. Learn the numbers.
Order of the check
Chock the wheels, drain the tanks, start the engine and build pressure. Check governor cut-out, then the build-up rate. Shut the engine off, release the parking brake, and do the static leak tests. Then test the warning device and the spring brakes, and finish with the visual inspection: slack adjusters, pushrod stroke, hoses, chambers, ABS lamp.
The numbers
- Build-up: 85 to 100 psi in 45 seconds or less.
- Applied leak test (engine off, parking brake released, pedal held one minute): no more than 3 psi loss single vehicle, 4 psi combination.
- Unapplied leak test (pedal released, one minute): no more than 2 psi single, 3 psi combination.
- Low pressure warning: must activate before pressure falls below 60 psi.
- Spring brakes: should apply at 20–45 psi.
Worked example
You hold the pedal for one minute on a tractor-trailer and the gauge falls 7 psi. Reasoning: the limit is 4 psi for a combination, so the unit fails the test - the loss could be in a chamber, hose, or the foot valve. Do not dispatch; find and fix the leak.
Traps
Running the leak test with the parking brake applied (the spring brakes are set, masking leaks and changing the reading), leaving the engine running, and reading the gauge before it stabilises. Also: checking pushrod stroke on one chamber and calling the whole vehicle good.