Cutting to Length
Determine the length of the brake line required, using stiff wire, solder, tape measure or the old brake lines as a pattern. Cut to the required length using a tubing cutter. Do not distort the brake line by over-tightening it in the cutter. File the ends to ensure it's square and remove burrs from the inside and outside edges. Blow filings out of the brake lines.
Selecting the Brake Line Nuts
If not reusing the original nut, be sure new nut matches with diameter, thread pitch, length, non-threaded lead, etc. Long style nuts are preferred for high vibration applications. Install nut on brake line before flaring.
Flaring Brake Lines
The appropriate single, bubble or double flare can be formed, using a good quality flaring tool such as the 007 brake line flaring tool. Serrated grips should not be used. Ensure that the dies grip the tube securely without deforming the tube section or denting its surface. Check that the flare is of the correct profile for the unit and component involved and that it has been formed symmetrically on the tube axis.
Installing the Brake Lines
Bend the nutted and flared tube carefully to shape, so that it will fit easily into position. Bends should be smooth and have as large a radius as possible. For tight bends on the larger sizes, a rolling die bending tool should be used. A minimum bend radius of three times the tube diameter is recommended. Tubing should not be kinked or strained into position. It should emerge cleanly from the nut without bearing against it. Lubricate the threads and the bearing surface of the flare with anti-seize compound to ensure that it and the nut will seat properly and the nut can be tightened without twisting the tube. Do not over-tighten the nut.
Tightening Torque
As a general rule from finger tight, continue tightening the nut until you feel it draw down tight, then tighten approximately 1/6 turn more. Do not over-tighten.
Installation Support
All brake lines should be supported at regular intervals along their length, using steel or plastic clips, each attached firmly to the body or chassis of the vehicle. When brake lines follow axle casings or suspension arms, which can induce vibration of the tube, it is essential to secure tightly. Tie wraps work well to secure to rear acle housings. Clips should be spaced at intervals of approximately 12 to 13 inches but no farther apart than those used for steel brake lines.
How to Identify Brake Line Nuts
The size wrench you use has nothing to do with identifying brake line nuts. Brake line nuts are more like hollow bolts and you measure the diameter of the threaded portion and the spacing of the threads.
American/British Thread Brake Line Nuts
In the U.S. inch system the standard fine thread 3/8" diameter bolt has 24 threads per inch (tpi). This may be called SAE, UNF or NF. A standard course thread 3/8" diameter bolt has 16 threads per inch. This may be called USS, UNC or NC. The United States, Canada and Great Britain have shared the Unified Thread Pitch system since World War II but older British threads are also measure in threads per inch. British threads are called BSF for fine threads and BSW or Whitworth for course threads. BS threads are not interchangeable with Unified threads.
Metric Thread Brake Line Nuts
Metric threads are measured by the diameter of the threaded portion and by how far apart the threads are. The common metric thread 10 x 1.00mm indicates a 10mm o.d. on the threaded portion and a thread pitch of 1.00mm which is one thread every millimeter. Since there are 25.4 millimeters in 1 inch, that works out to 25.4 threads per inch. A 10 x 1.25mm is a course pitch. With the threads 1.25mm apart that works out to 20.3 threads per inch. A 1.5mm pitch is a courser pitch that works out to 16.9 threads per inch.
Measuring Brake Line Nuts
A thread pitch gauge is needed to measure threads accurately. When measuring the diameter of a nut or bolt it will be slightly smaller that the stated size. 3/8 of an inch is .375" in decimal measurement. The actual measurement of the brake line nut would be .365". A 10mm brake line nut will measure 9.7mm or .382" of an inch.
How To Identify Brake Line Flares
Check that the flare is of the correct profile for the components involved. Look in the port where the brake line goes, if you see a volcano chape pointing up at you in the bottom of the port it is a 45 degree double flare. If the bottom is funnel shaped pointing away from you, it is a "bubble" or DIN flare.
Never use a single 45 degree flare with automotive nuts and fittings. Automotive brake lines are always a 45 degree double flare or a DIN (bubble) flare.
A 37 degree flare is acceptable for use with AN/JIC nuts and fittings with FedHill seamliess brake lines.
General Notes
Carefully check the thread pitch and diameter of nuts and fittings.
Note that the A1 and M% (illustration below) look very much alike and will crossthread.
Note that the M1 and P10 (illustration below) look very much alike and will crossthread.
Specifically, the A1 and P10 with 3/8 x 24 threads will thread into a 10mm hole but will fit loosely and probably fail when tightened or in service. A 10mm nut may start in a 3/8 hole but will strip immediately.
Note the lead (non-threaded) portion on M1 and P10.
Never use a fully threaded nut in place of a nut with lead. The threads will bottom out so that it feels tight but it's not.
Never use a nut with lead in place of a fully threaded nut.
There will not be enough threads engaged.
American and Asian vehicles typically us a double flare.
European vehicles may have DIN flare.
Late model vehicles can be any possible combination of nuts and flares.
For example a Ford Taurus can have 4 different nuts on the ends of the two rear brake lines!
For British/Girling/Dunlop users:
Note that old British fittings use the obsolete SAE convex (bubble) flare (shown in illustration below of P10) with a 45 degree backside angle. Beginning in the '70's the DIN (bubble) flare with a 90 degree backside angle (shown in illustration below of M1) was adopted. You can use the P10 with either the SAE/DIN bubble flare or SAE double flare. The DIN flare will work in older fittings that had the SAE convex flare form and eliminates the problem of the flare wedging the nut open causing you to curse as you try to remove your old brake lines. If you are re-using your original brake lines and are having difficulties in starting the nut into the fitting, examine the end of the carefully for swelling.
FedHill
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Where to buy brake lines
Where to buy brake line nuts
Where to buy brake line flaring tools