When the piston moves quickly back to its resting place, the brake fluid cannot return through the lines fast enough to avoid creating a low-pressure condition ahead "of it. The fluid must reach the low-pressure area in time for another stroke of the cylinder. To keep the system filled with fluid and to maintain a sealed system, fluid from the reservoir then enters the cylinder through a vent port to fill the void. Fluid also enters the system by flowing past the primary cups. During the return stroke, the edges of the cup pull away from the bore enough to allow fluid to pass around the piston assembly to the area of low pressure.