Which component is crucial for knowing the air pressure in the braking system?

The supply pressure gauge is essential for understanding the air pressure within the braking system because it provides a direct visual indication of the pressure level in the system. This information is crucial for maintaining safe braking performance, as both too much or too little pressure can lead to braking failure or compromised stopping power. The gauge allows the driver to monitor the system continuously and take necessary action if the pressure falls below safe levels, ensuring the vehicle remains operable and safe to drive.

Other components mentioned, such as the brake fluid reservoir, master cylinder, or brake light switch, do not provide information about the air pressure in the braking system. The brake fluid reservoir holds the hydraulic fluid used in braking systems (typically more relevant for air-over-hydraulic systems), while the master cylinder generates hydraulic pressure for the brakes. The brake light switch is related to the electrical system that activates the brake lights when the brakes are applied but does not directly indicate air pressure.