How is air pressure generally built up in an air brake system?

The process of building up air pressure in an air brake system primarily relies on the air compressor. When the engine is running, the compressor draws in ambient air and compresses it, creating high-pressure air that is stored in the air tanks. This stored pressurized air is essential for the operation of air brakes, as it allows for the rapid release and application of the brakes when pressure is modulated by the brake pedal.

The function of the air compressor is crucial because it ensures a continuous supply of compressed air, which can be quickly accessed by the brake system. This efficiency is vital for the performance and safety of air brakes in heavy vehicles, as it allows for better control and responsiveness during driving and stopping.

While factors like engine speed contribute to the efficiency of the compressor, they do not directly initiate the buildup of pressure. Similarly, hydraulic fluid does not play a role in air brake systems, which are fundamentally different from hydraulic systems. Manual operation can aid in certain brake systems but is not the primary method for generating air pressure in an air brake system. Thus, the compressor's role is fundamental, highlighting why it is the correct answer.