The brake master cylinder is the manual hydraulic pressure part of the brake system. The brake master holds hydraulic brake fluid that is used to actuate the brake system. Brake fluid flows through brake system components and then to the front brake caliper under pressure to slow the vehicle. When a master cylinder fails you may experience a low brake pedal, the master cylinder is divide into two separate sections, the primary and secondary pressure systems. This system was created inside the master cylinder for safety; if one half of the brake system fails you will still have the remaining half to help slow the vehicle. Note: You must know how to bleed a hydraulic brake system before you can service or change a master cylinder.