What should the air pressure gauge indicate in proper functioning?

In a properly functioning air brake system, the air pressure gauge should indicate that the pressure builds to a range around 120 PSI and then stabilizes. This indicates that the system is operating correctly and is within the safe working pressure for air brakes in commercial vehicles. This range is set to ensure that the brakes have sufficient air pressure to function effectively, providing the necessary stopping power and safety for the vehicle.

If the gauge indicates a pressure significantly lower than this range, it could signal a problem with the air supply or possible leaks in the system, which can impair braking performance. Conversely, readings above this range generally don't provide any additional benefit, as the system is designed to operate safely within that designated pressure window. Therefore, an indication of around 120 PSI is optimal for ensuring the air brake system functions effectively.