Brake Bleeder Screw.
Size: 8 x 1,25 mm

The bleeding process basically involves applying brake pressure, slightly loosening the screw - about 1/4 of a turn - to raise the end of the screw and allow air and fluid to flow into the hole at the bottom, through the screw and out the hole at the top. The screw is then tightened again and the brake lever released, drawing in more fluid from the reservoir and the process repeated until there is no more air left in the system.