Solid Brass 3/8-24 Female Inverted Flare Union For 3/16 Brake Lines. Unions are used to join two brake, fuel, or transmission lines together. This may be needed for the long brake line that runs for the front of the car to the rear axle. Use for joining 3/16" Domestic brake lines. Match the thread and flare type to the lines being connected. is important that both flare type and thread match for a leak-proof connection. SAE 3/16 tube size x 3/8-24 thread size