The brake master cylinder moves brake fluid under pressure to rest of the braking system. When you step on the brake, it is connected to the main plunger. As this is pushed into the brake master cylinder it acts on the components inside. The rear plunger is the first one to start moving. As it moves forward, brake fluid from the reservoir is sucked in through the fluid intake and return port. At the same time, fluid is sucked in through the equalization port. As the second circuit rear seal passes the intake and return port, it creates a fixed volume of fluid between the rear and front plungers. The more you step on the brake pedal, the more this brake fluid is now forced out into the second brake circuit to apply those brakes. At the same time, the pressure building up in this area overcomes the strength of the first circuit return spring and the front plunger (red) begins to move too. As with the rear plunger, it too sucks brake fluid from the reservoir until the first circuit rear seal passes the brake fluid intake and return port, trapping brake fluid between it and the front of the brake master cylinder. This brake fluid is then forced out into the first brake circuit, applying those brakes. When you take your foot off the brakes, the return springs push the plungers back into their neutral position. Fluid returns to the brake fluid reservoir and the system goes back to an un-pressurized state.
