Air Brake System Checks (Leak, Low Pressure Warning, Spring Brake, Service Brake)
What the exam tests
This domain tests the four operational checks and their pass criteria. You need to know the correct order, when the engine is running or off, and what each result proves.
Static leak test
With a fully charged system and the engine off, release the parking brake, apply the service brake, and hold for one minute. Loss should not exceed about 3 psi in a single vehicle. Then apply the parking brake and hold for one minute; loss should again be within the limit. Excessive loss means find and repair the leak.
Low-pressure warning test
With the engine off and key on, pump the brake pedal to reduce pressure. The warning light and buzzer must activate before the gauge falls below 60 psi.
Spring-brake test
Continue reducing pressure. The spring brakes should apply at roughly 20-45 psi and hold the vehicle. If they do not apply or do not hold, the vehicle is unsafe.
Service-brake test
At low speed, apply the service brake firmly. The vehicle should slow and stop straight without pulling. A pull or delayed stop needs inspection.
Worked example
A warning sounds at 55 psi and the spring brakes set at 30 psi. Those results are within normal ranges. If the warning first sounds at 40 psi, the check fails even if the spring brakes later apply.
Traps that cost marks
Do not run the leak test with the engine on. Do not leave the parking brake applied during the service-brake leak test. Do not confuse the low-pressure warning test with the buildup test. A warning at the wrong pressure is a failure even if the buzzer works.