Vehicle Systems and Malfunctions
Air brakes
Know the air system cold. The compressor builds pressure, the governor cuts out near 120–125 psi and cuts back in near 100 psi, and the system should build from 85 to 100 psi within about 45 seconds at operating rpm. With the engine off and brakes released, leakage should not exceed about 3 psi per minute on a single vehicle or 4 psi on a combination; with brakes applied, about 4 psi per minute. The low-air warning must come on by 60 psi, and spring brakes apply somewhere between roughly 20 and 45 psi.
Other systems and warning signs
Watch coolant temperature, oil pressure, and the voltmeter. A rising temperature, falling oil pressure, or a battery light means stop and investigate. An ABS lamp that stays on means the ABS is disabled - you still have normal brakes, but no antilock protection, so adjust your driving and get it checked. Listen for air leaks, and check that the parking brake holds.
Traps
Questions suggest the ABS light means the brakes are gone, or that the warning buzzer can be ignored until the truck stops moving on its own. Another trap is treating leakage limits as one number for all configurations.
Rule of thumb
If the low-air warning sounds while moving, stop in a safe place immediately. Below roughly 45 psi the spring brakes will begin to set on their own.