What type of fittings are used on brake lines?
Brake lines typically use flare-style fittings—most commonly bubble or double flare—that seal against a conical seating surface inside the corresponding nut. In higher-performance or aftermarket builds, you may encounter AN fittings and braided hoses, and caliper connections often use banjo fittings. This article breaks down the main types you’re likely to encounter on street and track cars.
Common brake-line end fittings
Most stock brake lines use flared ends with a matching flare nut to seal the connection. Here are the two primary styles you'll see on passenger vehicles today.
- Bubble flare ends: One-flare style that forms a small, rounded dome when flared; still widely used on many older and some current vehicles.
- Double flare ends: A two-fold flare that creates a stronger seal and is standard on a wide range of modern vehicles.
Both styles rely on a flare nut that threads onto the prepared tube end and clamps the seating surface to create a hydraulic seal under high brake pressure.
High-performance and aftermarket options
When upgrading lines for performance or racing, enthusiasts often opt for fittings designed for higher pressure and repeated flexing, including braided hoses and different seating angles. The main options are:
- AN fittings and braided hoses: Internationally standardized 37-degree flare fittings (often labeled as -3, -4, -6, etc. for size) paired with braided stainless hoses. These are common in performance builds and are compatible with many master cylinders, calipers, and hoses when matched correctly.
- Banjo fittings and caliper connections: Calipers and some master-cylinder outlets use banjo-style connections, secured with a banjo bolt and washers; lines may end in a banjo-style tube connection or be adapted to AN or flare ends.
- Metric brake-line fittings: European and some Asian vehicles use metric thread sizes and banjo bolts; adapters are available to connect these to standard hoses or flare ends when performing restorations or upgrades.
In all cases, ensure the fittings match the system’s port sizes and seating surfaces, and use the correct torque and sealants or washers prescribed by the manufacturer to prevent leaks.
Connecting components and maintenance
Brake-line fittings must seal properly at every connection, from the master cylinder to the wheel/caliper. Always replace worn nuts, washers, and seals; torque to the vehicle manufacturer’s specification; and use appropriate anti-corrosion coatings or dielectric grease where recommended. Incompatible fittings can lead to leaks, reduced braking effectiveness, or failure under pressure.
Caliper and master-cylinder connections
Banjo bolts and banjo-style fittings attach hoses to calipers, while flare or AN adapters connect lines to the master cylinder and ABS modules. When mixing brands or upgrading lines, verify that the thread sizes, seat angles, and nut dimensions are compatible to maintain a proper seal.
Summary
Brake-line fittings are predominantly flare-based, with bubble and double flare as the standard end styles. For performance or custom builds, AN fittings with braided hoses are common, and caliper connections often employ banjo fittings. European and other markets may use metric fittings, which require appropriate adapters. Across all types, correct sizing, seating, torque, and compatible materials are essential for safe brake operation.
What type of thread are brake line fittings?
Brake line fitting thread sizes vary, but common SAE sizes include 3/8−243 / 8 minus 243/8−24, 7/16−207 / 16 minus 207/16−20, 7/16−247 / 16 minus 247/16−24, 1/2−201 / 2 minus 201/2−20, 9/16−189 / 16 minus 189/16−18, and 9/16−209 / 16 minus 209/16−20. For metric vehicles, common sizes are expressed as diameter followed by thread pitch, such as M10x1cap M 10 x 1𝑀10𝑥1 (for a 10mm diameter and 1mm pitch). It's crucial to identify the correct size for your specific vehicle by checking the original brake lines or using a thread gauge.
Common SAE (inch) brake line fitting thread sizes
- 3/8"×243 / 8 " cross 243/8"×24
- 7/16"×207 / 16 " cross 207/16"×20
- 7/16"×247 / 16 " cross 247/16"×24
- 1/2"×201 / 2 " cross 201/2"×20
- 9/16"×189 / 16 " cross 189/16"×18
- 9/16"×209 / 16 " cross 209/16"×20
- 5/8"×165 / 8 " cross 165/8"×16
Common Metric brake line fitting thread sizes
M10 \times 1.0 (commonly for 3/16" line), M10 \times 1.5, and M12 \times 1.5.
How to identify the correct size
- Check your existing brake lines: Carefully inspect the fittings on your vehicle's current brake lines for any size markings.
- Use a thread gauge: A brake line fitting size gauge is a specialized tool with both male and female ends to help you quickly identify common thread sizes, say Amazon.com.
- Consider your vehicle's origin: For European or some Asian vehicles, it's highly likely the fittings will be metric, while American vehicles are more likely to use SAE/inch fittings.
- Match line and fitting sizes: Be sure that both the line size (3/16",1/4"3 / 16 " comma 1 / 4 "3/16",1/4") and the thread size are compatible to ensure a secure and leak-free connection.
Are all brake line fittings the same?
There are several types of brake line fittings, each designed for specific applications. Understanding the differences between these fittings is crucial for selecting the right type for your vehicle: Double Flare: Used in most automotive applications, double flare fittings are known for their durability and secure fit.
Should you use compression fittings on brake lines?
No, you should not use compression fittings on brake lines because they are dangerous and can lead to brake failure. Brake lines require fittings specifically designed to handle high hydraulic pressure, and standard compression fittings are not built for this purpose and can leak or rupture. Using them is often illegal and can cause a vehicle to fail inspections.
This video explains why compression fittings are unsafe for brake lines: 50sThe Car Care NutYouTube · Jan 1, 2025
Why compression fittings are unsafe for brake lines
- Pressure limits: Brake systems can operate at very high pressures, and most compression fittings are not rated to handle this stress and are prone to failure under such conditions.
- Leak risk: The seal that a compression fitting creates can be compromised by the high pressure, leading to a loss of brake fluid and potential brake failure.
- Legality: Many states have regulations that explicitly prohibit the use of compression fittings on brake lines, and a vehicle with them will likely fail inspection.
- Safety hazard: Since brakes are a critical safety system, using a weak or improper connection is a significant liability and a serious safety risk to the driver and others.
This video demonstrates how compression fittings work on brake lines: 1mbackyardmechYouTube · Jun 23, 2014
What to use instead
- For repairs that require joining brake lines, use professional-grade double-flare fittings or a pre-bent, factory-style brake line.
- If you've had compression fittings installed, have them replaced with the correct fittings by a professional to ensure your braking system is safe.
This video shows how to replace a brake line with a compression fitting: 1mFerKurlYouTube · Sep 3, 2020
What type of fittings are used for brake lines?
What are the main types of brake fittings? The main types include double flare, bubble flare, and AN fittings. Each type is designed for different applications and connections within the brake system.
