At last there is now a hydraulic brake bleeding system where seeing is believing. This vacuum bleeding device consists of a transparent cap that forms a tight seal to the master cylinder and has a fitting to attach a vacuum pump. After the brake system has been filled with fluid and primed, the bleeder cap is installed on the master cylinder and the vacuum pump is applied. Under a vacuum of 30 to 40 inches Hg (a pressure differential of about 15 to 20 psi) air will be drawn out of the system and the bubbles will be visible through the transparent cap. When the bubbles subside, there is no more air in the system. This is especially advantageous with ABS and dual disc brake systems. It is also a cleaner way to purge the system of air with less fluid used and a reduced chance of cosmetic damage due to brake fluid spillage. Works with Sonic over & under pressure kit.