Which process closes the passage between the compensation port and the master cylinder bore?

The process that closes the passage between the compensation port and the master cylinder bore happens when the primary pistons move forward.

In a hydraulic brake system, when the driver applies the brake pedal, the master cylinder generates hydraulic pressure, causing the primary pistons to move toward the calipers. As these pistons advance, they block off the compensation port, which is designed to maintain fluid levels and allow for fluid movement within the master cylinder. Closing this port is essential because it helps prevent excessive leakage and ensures that the brake system maintains adequate pressure for effective braking. It also allows for the product of the hydraulic force generated to be directed precisely to the brake calipers, enhancing their performance.

While other processes may affect the system, such as the driver releasing the brake pedal, the activation of the calipers, or the delivery of fluid pressure, the specific action of the primary pistons moving forward is what directly leads to the closing of the compensation port in the master cylinder.