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.
If there is a 10psi (69kPa) brake application made, and a slight leak exist in one part of the system that causes a reduction in the 10psi brake application, the foot valve will reopen and top up the air pressure that part of the system.
In other words, even if there is a leak, the foot valve and other valves will compensate for this by maintaining a 10psi brake application pressure.
The brake pedal is a valve. It is called either the foot valve or the treadle valve.
Learn about the parking brake here!