What is the size of a brake line?

Three-sixteenths inch and one-quarter inch are the most common sizes of brake lines. Most cars and trucks manufactured use a three-sixteenth inch tubing, as of 2015. Many vehicles built in the 1950s use one-quarter inch lines as do some late model General Motors, Japanese and European manufactured cars and trucks.

Part specifications for brake lines list three-sixteenth lines as CNF-3 and one-quarter inch lines as CNF-4. Vehicles can utilize both sizes with the one-quarter inch lines being used in shorter high-pressure segments such as between the master cylinder and the anti-lock brake system pump, or long runs to the rear of full-size cars and trucks.