Final answer:
In an air brake system, the air compressor should start pumping at around 120 psi of air pressure.
Step-by-step explanation:
In an air brake system, the air compressor should start pumping at around 120 psi (pounds per square inch) of air pressure.
The air compressor is responsible for compressing the air and supplying it to the rest of the system.
Once the air pressure reaches the required level, the compressor will automatically start pumping and maintain the pressure within the system.