In the brake system, pressure is generated when which action occurs?

Pressure in the brake system is generated when the driver applies the brake pedal. When the pedal is pushed, it allows the master cylinder to exert hydraulic pressure on the brake fluid. This pressure is transmitted through the brake lines to the brake calipers or wheel cylinders, causing the brake pads or shoes to engage with the rotors or drums, ultimately slowing or stopping the vehicle.

The other options do not directly cause pressure to be generated in the braking system. The wearing down of brake pads, for instance, may affect braking performance and could lead to the need for maintenance, but it does not create pressure. Similarly, while heat can build up in the brakes during operation due to friction, it does not generate hydraulic pressure itself. Lastly, the evaporation of brake fluid would compromise the braking system, leading to a lack of pressure rather than its creation. Thus, the action that specifically initiates the generation of pressure in the brake system is the application of the brake pedal by the driver.