Which of the following is NOT part of the supply circuit in an air brake system?

In an air brake system, the supply circuit is responsible for generating, storing, and supplying compressed air to enable the braking process. The components that are part of this supply circuit typically include the air compressor, which generates compressed air, the air tank that stores this air for use when required, and the relay valve that helps regulate the distribution of air to the brake chambers.

The brake pad, on the other hand, is not part of the supply circuit. Instead, it is a component of the braking mechanism itself, which works in conjunction with the brake chamber to create friction against the brake drum or disc, ultimately slowing down or stopping the vehicle. This distinction is crucial as the supply circuit focuses on the generation and management of compressed air, while the brake pad directly contributes to the physical braking action.