Dual Circuit Architecture: Why Two Independent Systems?
A dual air brake system uses two completely separate air circuits — the primary (rear) circuit and the secondary (front) circuit — each with its own reservoir, pressure protection valve, and brake circuits. The purpose is fail-safe redundancy: if one circuit loses pressure due to a line rupture or component failure, the other circuit can still stop the vehicle. The air compressor (driven by the engine) fills both reservoirs through the wet tank, which also contains the compressor governor. The governor cut-in pressure is approximately 100 psi and cut-out pressure is approximately 125 psi — these are the values tested on the CDL exam. As both reservoirs fill above 60 psi, the low-pressure warning light should extinguish. The supply pressure protection valve prevents the trailer supply circuit from draining the tractor's reservoirs if the trailer develops a catastrophic air loss. During a pre-trip inspection, the driver must build air to governor cut-out (approximately 120–125 psi), then turn off the engine and observe the pressure gauges for at least one minute — a pressure drop of more than 3 psi per minute with brakes released, or more than 4 psi per minute with brakes applied, indicates a system leak requiring service before operation.