All hydraulic brake systems operate on the same basic principle. When the operator moves a brake pedal or other brake operating control, the movement is transmitted to a master cylinder or to a power brake control valve from which fluid pressure is delivered through connecting lines to a brake assembly connected to a wheel or shaft whose movement is to be braked. The fluid pressure acting on the brake assembly pushes brake linings Into contact with surfaces of a rotating disc. The resulting friction slows--and finally stops--the continued rotation of the wheel or shaft to which the disc is connected. When the brake pedal or brake control is returned to the off position, brake operating pressure is relieved, the brake lining loses contact with the disc, and the wheel or shaft is free to turn again. Aircraft wheel brake systems are dual in nature in that they are composed of two identical subsystems that can be operated independently of each other to provide separate braking action for the landing gear on each side of the aircraft. Each subsystem is operated by a brake pedal that is hinge-mounted to the top of the aircraft rudder pedal. Since each brake pedal can be operated independently the brakes can be used for steering the aircraft. A list of components, which may be found in varying combinations to make up the different wheel brake systems, includes the following: master cylinder (or a power brake control valve), wheel brake assemblies de-boosters, parking brake valves, shuttle valves, accumulators, connecting lines, and bottles charged with compressed air. The minimum number of parts which could be used to perform the function of a simple wheel brake system are a master cylinder (or a power brake control valve), a wheel brake assembly, and connecting lines.