There are about 5 or 6 different ways to bleed the brake system, all of them require the reservoir to be kept "full". The best way to flush the system is with a pressure bleeder. Gravity bleeding takes forever! Vacuum bleeders sometimes work. Using a second can with fluid works O.K. Having one person pumping while another operates the bleeder valve takes a while. Whatever method you use BE Careful! brake fluid can be toxic!