In a dual air brake system, which axle brakes does the 'primary' circuit typically operate, and which does the 'secondary' circuit typically operate?

AAMVA CDL Manual, Section 5.2 (Dual Air Brake), p. 5-5. (49 CFR 383.111(b)(1) requires knowledge of 'general air brake system nomenclature' as a topic area but does not itself specify which axles the primary/secondary circuits control; that specific fact

This redundancy is a core safety feature: if one circuit fails, the other can still brake part of the vehicle. Understanding which axles each half controls helps a driver correctly diagnose reduced stopping power if one system's pressure drops.