The purpose of bleeding the brakes is to expel air trapped in the hydraulic system. The system must be bled whenever the pedal feels spongy, indicating that compressible air has entered the system. It must also be bled whenever the system has been opened or repaired. You will need a helper for this job. You may need to bleed the hydraulic system at all 4 wheel positions if air has entered the system due to low fluid level, or from brake lines having been disconnected from the master cylinder. If a brake line is disconnected from one wheel, only that wheel caliper/cylinder needs to be bled. If lines are disconnected at any fitting located between the master cylinder and the wheels, then the brake system served by the disconnected line must be bled.