Brake Systems
This domain covers air brake system operation, adjustment, component inspection, and ABS. It is the highest-risk system on the trailer, so questions test both theory and measurement discipline.
Air system flow
Compressor feeds the governor, which controls cut-in and cut-out pressure, then the reservoirs. The foot valve signals the relay valve, which applies service chambers. The trailer receives supply (emergency, red) and service (blue) air through glad hands; the emergency relay valve applies spring brakes if supply pressure is lost.
- Leak testing: build pressure, shut down, and observe gauge drop against CVSA or OEM limits, both released and applied.
- Glad hand seals and filter screens are frequent leak sources.
Adjustment and inspection
Pushrod stroke is measured at normal operating pressure with the brakes fully applied, comparing the mark against the limit for that chamber size and type. Automatic slack adjusters should not be manually adjusted to compensate for chronic over-stroke - that hides a worn S-cam, bushing, or damaged chamber. Inspect lining thickness, drum diameter and scoring, cracks, and contamination.
ABS
Wheel speed sensors, tone rings, modulator valves, and the ECU. Sensor gap and tone-ring condition matter; blink codes and scan tools retrieve faults. A wheel-end repair can trigger a sensor code if the tone ring is disturbed.
Worked example
A trailer's spring brakes apply whenever the tractor is shut off. Reasoning: the emergency relay valve applies parking brakes on loss of supply. Check glad hand seals, supply hose, and the relay valve for leakage or a stuck valve. The fault is on the supply side, not the service side, so adjusting slack adjusters would be wasted work.
Traps that cost marks
Caging a spring brake and leaving it caged, adjusting an automatic slack adjuster instead of diagnosing, measuring stroke at the wrong pressure, and ignoring oil-contaminated linings that will fade. Also remember that a leaking diaphragm can pass air internally and cause drag.