The master cylinder reservoir should always hold enough fluid to allow the brake pads to completely wear away without the risk of running out of fluid. As a disc brake pad wears, the caliper piston will extend out of its bore. As the caliper piston extends, the master cylinder fluid level will drop in order to fill the caliper bore with brake fluid. If the reservoir is too small, there is a possibility that the master cylinder can run out of fluid and the brakes will fail.
