What closes the passage to the compensation port when brakes are applied?

The master cylinder pistons play a crucial role in the functioning of the braking system, specifically in relation to the compensation port. When the brakes are applied, the pistons in the master cylinder move forward, generating hydraulic pressure. This action effectively pushes against the fluid in the brake lines, which, in turn, pushes the brake shoes against the brake drum or rotor to create friction and slow down or stop the vehicle.

As the master cylinder pistons travel forward under pressure, they also cover or close off the compensation port. The purpose of this port is to allow for the equalization of pressure and fluid between the master cylinder and the brake system when not engaged. When the pistons block this port during brake application, it helps to maintain constant hydraulic pressure within the system, ensuring that the brakes engage effectively without any loss of pressure caused by backflow or fluid leakage.

In summary, the closing of the compensation port is a result of the master cylinder pistons' movement, which is integral to the effective operation of hydraulic brake systems.