Brakes, Air Systems, and Wheel-End Components
Air system tests in a fixed order
Air brake questions reward a disciplined sequence. With the system fully charged and the engine off, release the parking brake and watch the gauge for one minute; leakage on a single vehicle should not exceed 3 psi (4 psi for a combination). Then apply and hold the service brake and read again, with the same limits. Next, build-up: at fast idle the compressor must raise pressure from about 85 to 100 psi within 45 seconds, and the governor must cut out at the OEM-specified pressure and cut back in roughly 20 psi lower. Finally, test the warning: the low-pressure warning must activate before pressure falls below 60 psi, and spring brakes must apply in the 20 to 45 psi range.
Then inspect the foundation brakes. Measure pushrod stroke against the limit stamped on the chamber or listed in the OEM chart; a stroke beyond limit is an out-of-service condition. Automatic slack adjusters are not to be manually adjusted out of a problem; if stroke is long, find the cause, such as a backed-off adjuster, bent pushrod, or broken chamber. Friction lining minimum is typically 1/4 inch at the shoe center.
Wheel-end work covers hub oil level and leaks, wheel bearing play, ABS tone rings and sensors, drum or disc condition, and lug torque with a calibrated wrench.
Worked rule of thumb: chock, then drain. Moisture in the wet tank means the air dryer is saturated, and daily tank draining plus dryer service prevents most winter brake failures.