Dual Air Brake System and Air Supply
Two circuits, one supply
A dual (split) system divides braking into two independent circuits so a single leak cannot take out all braking. Each circuit has its own reservoir, check valve, and low-pressure warning; typically one circuit serves the front axle and the other the rear axle(s), and the parking/emergency circuit is charged separately. Check valves keep a failed circuit from draining the healthy one, and the supply reservoir (wet tank) feeds the primary and secondary tanks through one-way valves.
Compressor, dryer, and tanks
Air flows compressor → dryer → supply tank → primary/secondary tanks → valves and chambers. The dryer purges moisture each cycle; a failed purge leaves water in the tanks, which can freeze or corrode valves. Reservoirs carry drain cocks at the low point, and a safety valve protects each tank. The compressor must build pressure from 85 to 100 psi within a specified time (commonly about 45 seconds at governed engine speed for a single vehicle), and the system must hold pressure with the engine off and brakes released.
Traps and a rule of thumb
Questions test three ideas repeatedly. First, the dual system is not two complete brake systems - each circuit still shares the foundation brakes on its axles. Second, a leak in one circuit must not drain the other; that is the check valve's job, not the governor's. Third, the low-pressure warning must work for both circuits, and the buzzer should sound before 60 psi. A common wrong answer says the governor isolates a leaking circuit - it does not.
Rule of thumb: build time 85→100 psi about 45 seconds; static leak test - pressure drop should be less than 3 psi in one minute with brakes released (single vehicle), and less than 4 psi with brakes applied.