Final answer:
The piston inside the caliper of a disc brake system is often made of aluminum or steel.
Step-by-step explanation:
The piston inside of the caliper in a disc brake system is often made of a metal alloy called aluminum or steel. These materials are chosen for their strength, durability, and resistance to heat and corrosion. The piston transfers the force applied to the brake pedal to the brake pads, which then press against the rotors to slow down or stop the vehicle.