Dual Air Brake Systems and Safety Features
Two circuits, one pedal
A dual air brake system splits into a primary circuit (rear brakes) and a secondary circuit (front brakes). The foot valve has two separate sections fed by separate reservoirs, so a leak or ruptured line in one circuit cannot drain the other. If one circuit fails, the other still applies its brakes and the low-pressure warning (light and buzzer) tells you so - but expect longer stopping distance and a pull toward the good side.
Check valves stop air from flowing backward out of a healthy circuit into a failed one. On a combination vehicle the tractor protection valve does the same job between tractor and trailer: it closes automatically when trailer supply pressure drops into roughly the 20–45 psi range, keeping the tractor's air from escaping through a broken trailer line. The driver controls it with the trailer air supply valve, the red octagon knob - push in to charge the trailer, pull out to shut off supply and set the trailer spring brakes.
Safety features
Spring brakes are the parking and emergency brake: a heavy spring applies them and air pressure holds them off, so they set automatically as pressure falls and cannot be released until pressure is rebuilt. ABS is a separate electronic system and does not change how you brake - keep steady pressure and let it work. Older tractors may carry a front brake limiting valve that reduces front braking in slippery conditions.
Traps: the red octagon is trailer air supply; the yellow diamond is the parking brake. And a dual system is not a licence to keep driving after a circuit fails - you pull over.