Final answer:
The air compressor in a well-maintained air brake system typically starts pumping at about 125 PSI. This is the general 'cut-in' pressure for many vehicle air brake systems.
Step-by-step explanation:
For air brake systems in good condition, the air compressor should start pumping at about 125 PSI. Air brake systems require a compressor to generate the necessary pressure to operate the brakes effectively. The compressor usually kicks in at a preset pressure level, known as the "cut-in" pressure, to maintain the air system's required pound per square inch (PSI). While this figure can vary depending on the specific vehicle and manufacturer, a typical activation pressure is around 125 PSI for many vehicles' air brake systems.