The wheel farthest away from the master cylinder is bled first which in most cases is in this order: Right Rear, Left Rear, Right Front, Left Front. Failure to bleed in the proper order will cause air to remain in the lines. This is because if you bleed a short line, (ex. front lines) before bleeding a long line (ex. rear lines), the fluid in the short line will compress and prevent enough pressure from being built up in the long line to expel the air. Furthermore, incorrect bleeding order will cause pressure to build up on one side of the proportioning valve, causing it to close on the low pressure side making it impossible to force fluid to the rear lines. If this happens you will never be able to bleed the system until you readjust the proportioning Valve.