On dual air brake systems, what is referred to as the primary system?

In a dual air brake system, the primary system is the one responsible for the most critical braking; this typically includes the front brakes. Specifically, the primary system is designed to engage and control the braking forces for the front wheels, ensuring that adequate stopping power is available when needed. The front braking system is crucial for maintaining control and stability while stopping, as the weight transfer during braking shifts the vehicle's load forward.

The rear braking system and related components play a supportive role in the overall braking functionality but are not categorized as the primary system. The emergency brake system, while critical for safety, functions independently and serves as a backup rather than the main system. The antilock brake system is a safety enhancement that helps prevent wheel lockup during hard braking but does not define the primary braking system itself.