The brake pedal applies the service brakes - truck, or truck and trailer brakes. In other words, the brake pedal brings the vehicle to a stop when it it depressed.
The foot valve operates the secondary and primary sub systems simultaneously.
Failure in one part of the dual air system does not affect the operation of the other, owing to the division of the braking system into two independent subsystems.
The brake pedal valve is also a self-balancing valve.