How does a break master cylinder work? A brake master cylinder is responsible for taking the energy that the driver exerts on the brake pedal and turning it into braking force at the wheels. A brake master cylinder is comprised of the main body which has a piston that slides within it and a reservoir. The reservoir may be cast into the body or a separate piece made out of plastic but the job of the reservoir is the same, it keeps excess brake fluid in reserve for when the brakes are used. When the pedal is pressed, a linkage pushes on the piston that is in the bore of the master cylinder and that piston compresses the brake fluid which is pushed through the brake lines to the individual slave cylinders. These slave cylinders exert pressure against the brake pads which either ride against a break rotor or a brake drum depending on whether a disc or drum braked car and the friction between the brake pad and the rotor or drum is what causes the vehicle to slow or stop. Most modern vehicles have power brakes. In most cases this power is supplied via a vacuum diaphragm in the brake booster. This brake booster receives vacuum from the manifold and transfers that vacuum into additional pressure on the master cylinder. The net result is that you have less effort required at the pedal and better braking power.