Abstract: A tool for venting hydraulic systems comprises a tubular tool element which forms a polygonal wrench at a first end. A flexible drain tube extends through the tubular tool element. An end of this drain tube is adapted to be pushed on the nipple of a venting valve. A metal sleeve is located in the flexible drain tube, and a clamping device is provided on the tool element adjacent the metal sleeve for clamping the flexible drain tube to the tool element. This facilitates the assembly and use of the tool. With such a tool, a venting valve is unscrewed to a limited extent through the polygonal wrench such that air is allowed to be vented from the system. The venting valve has a nipple. A flexible drain tube is placed on this nipple. The flexible drain tube prevents uncontrolled outflow of hydraulic liquid from the hydraulic system, thereby preventing contamination of the environment. Instead the hydraulic liquid is guided into a collecting reservoir.