System layout beats component knowledge
T4 is a system-thinking exam. You need to know how the compressor, governor, wet tank, dry tank, foot valve, relay valves, quick-release valve, spring brakes, and treadle valve all interact. A leak at a specific fitting produces predictable pressure drop patterns on the dash gauges. Learn those patterns.
The 90/120 rule and buildup time
Federal Motor Carrier Safety Regulations require the compressor to build pressure from 85 to 100 psi in 45 seconds or less at 600 to 900 engine RPM. Know the pressure ranges, the governor cut-in and cut-out settings (typically 100 to 125 psi), and the max allowable leakdown for both single vehicle and combination vehicle static and applied tests.
ABS on heavy vehicles
Heavy truck ABS uses modulator valves separate from the ABS ECU, unlike passenger cars. Know that a wheel speed sensor code on a trailer axle can be caused by an air gap issue that only appears with vehicle dynamic loading. Field-adjustable sensors are common.