In the newest designs, each subsystem routes fluid to diagonally opposite wheels rather than just the front or back wheels. When the brake pedal is not engaged, a vacuum surrounds the large diaphragm. Depressing the pedal opens a valve that allows air to enter on one side of the diaphragm, restoring atmospheric pressure. The pressure difference between the two sides then forces the diaphragm inward, actuating pistons in the master cylinder. This action boosts power to the hydraulic system.
Some newer cars have disc brakes on all four wheels.
The cylinder's two pistons force the shoes outward toward the drum when the pedal is applied. The linings on the brake shoes push against the inner surface of the drum. The resulting friction stops the wheel from turning.