Brake Line Fittings & Sizing Guide: Tube Nuts, Unions, Adapters & Line Sizes
Nothing stalls a brake job like the wrong fitting or the wrong line size — the parts store round-trip every DIYer knows too well. This pillar guide gathers our Knowledge Garage articles on brake line fittings and sizing so you can identify what your vehicle needs the first time.
Pick the right line size
Start with diameter. What Size Brake Line Do I Need? covers the common sizes (3/16" and 1/4" for brakes, larger for fuel and transmission), how to measure what's on your vehicle, and when size actually matters.
Understand fitting types
Understanding Brake Line Fittings: Tube Nuts, Unions & Adapters Explained breaks down the three fittings you'll touch on every job — what each does, and how they work together with the flare to seal the system.
Match fittings to your project
For the full picture — thread sizes, metric vs. SAE, and choosing fittings for a complete re-line — work through Fitting Your Project Together: A Guide to Brake Line Fittings. Remember that fitting type and flare type must match; if you need a refresher, our Brake Line Flaring Guide covers it.
Shop the parts
Browse all brake line fittings — or jump straight to tube nuts, unions, adapters, or an assortment kit that covers the common sizes in one box. Not sure what thread you have? A fitting gauge ends the guesswork.
Frequently Asked Questions
What size brake line do most cars use?
3/16 in. (CNF-3) is standard on nearly all modern cars and trucks. 1/4 in. appears mostly on 1950s-era vehicles and some heavier truck and trailer applications.
Are brake line fittings universal?
No. Thread size, thread pitch, and flare type all vary by vehicle. A fitting must match both the thread of the component and the flare on the line.
What is the difference between a tube nut and a union?
A tube nut secures the flared end of a line into a component such as a caliper, wheel cylinder, or master cylinder. A union joins two sections of line together end to end.
How do I tell metric from SAE fittings?
You often can't by eye - the sizes are close enough to cross-thread. Use a thread gauge or a brake line fitting gauge to identify the thread before you buy.
Related guides
Continue with our Brake Line Replacement Guide, Brake Line Flaring Guide, and Brake Line Materials & Corrosion Guide.