Compression unions are used in low pressure application such as: air, oil, and water. They are generally used to connect lengths of copper, brass, and aluminum tubing. To make connection, place the nut and then the sleeve over the tubing and then insert tube into the fitting body. When tight, the wedging action of sleeve between body and nut create a leak free connection. Use for joining 3/8in copper, brass, or aluminum tubing in low-pressure applications (NOT brake lines).