Standard disc brakes on all four wheels need a backup. If the primary hydraulic system fails completely, you need a separate mechanical link to stop the car. Most vehicles rely on a simple steel cable to pull the emergency brake mechanism into action.
Some manufacturers integrate a small drum brake directly into the rear wheel hub. This drum is exclusively for emergency use. It has no hydraulic lines. The cable pulls it directly. There is no fluid involved in this specific braking action.
Other systems use a lever connected to a screw or cam. This mechanical action presses the disc brake piston. It bypasses the master cylinder entirely. The result is a purely mechanical stop. You don’t need brake fluid to halt the vehicle.
