Which of these is not part of a simplified service brake circuit?

The simplified service brake circuit primarily consists of components that play a direct role in the functioning of the braking system. In this context, the pressure gauge is not a fundamental component of the simplified service brake circuit. While a pressure gauge is important for monitoring system pressure and ensuring that the air brake system operates within safe limits, it does not actively engage in the braking action.

The brake pedal is crucial as it acts as the primary interface for the driver to apply the brakes. The air lines are essential for transporting compressed air throughout the braking system, and the brake chambers convert the air pressure into mechanical force to apply the brakes. Each of these components is integral to the overall functionality of the service brake circuit, while the pressure gauge primarily serves a monitoring purpose, making it the element that stands apart from the core components involved in the braking process.