Final answer:
Hydraulic pressure is what causes the secondary piston to move and develop pressure in the secondary circuit of a brake system. The force applied by the driver on the brake pedal is amplified through a lever and the hydraulic system, resulting in increased pressure on the secondary piston.
Step-by-step explanation:
In a hydraulic brake system, the secondary piston moves and develops pressure in the secondary circuit due to hydraulic pressure. When the driver pushes the brake pedal with a certain force, it increases through a lever and amplifies further through the hydraulic system. This increased pressure is transmitted to the secondary piston, causing it to move and generate pressure in the secondary circuit.