Air Brake Systems
Air brake fundamentals
Air brakes are fail-safe and pressure-balanced: spring brakes apply when pressure is lost, service brakes apply when pressure is admitted. Trace the system in order - compressor, governor, air dryer, reservoirs with check valves, foot valve, relay valve, quick-release valve, brake chambers, slack adjusters. The governor cuts the compressor in and out (commonly cut-out near 120–135 psi with cut-in 15–25 psi lower); the safety valve protects the system near 150 psi; low-pressure warning must operate by 60 psi; spring brakes begin applying automatically at roughly 20–45 psi.
Know the required tests. Static leak-down: at full pressure with the engine off, measure released and then applied leak rates against the applicable standard (commonly around 2 psi/min released and 3 psi/min applied). Build-up time from 85 to 100 psi is measured at fast idle. Pushrod travel is measured with the brakes fully applied and compared to the chamber's limit chart - excessive travel means the slack adjuster is out of adjustment or a component is worn.
Worked example. A tractor loses pressure faster than the limit with the engine off, and you hear a faint hiss at the rear axle with the brakes released. Isolate by capping lines and watching the gauges. A leak that stops when a valve is capped tells you the fault is downstream of that valve. Automatic slack adjusters should be checked, not routinely cranked up - if one is out of adjustment, find out why.
Also know the tractor protection valve, inversion valve, alcohol evaporator, and why brake fade and imbalance cause pull and stopping-distance failures.