How to Fix Leaky Brake Line Fittings

How to Fix Leaky Brake Line Fittings

posted in: Hard Line Sealing | 0

Is your brake line leaking where the flare joins the fitting? If so, it is important to address the issue immediately and with a legitimate solution.

When it comes to leaky brake line fittings there are plenty of band-aid fixes that could end up being disastrous. I was fed up with these unreliable methods when I was attempting to repair a leaking brake line in my Packard Coupe, and that is when I began looking for a real way to solve this issue.

Why Brake Lines Leak at the Flare

Brake lines will leak at the flare when there is a poor connection with the seat of the fitting. This type of leak is commonly known as a weep. It can be caused by a ding in the fitting, but most of the time it is the result of a tiny imperfection in the flare that occurs while flaring the tube. No matter what flaring tool I used, I never got a clean, concentric finish that enabled the flare to seat with the fitting. And it didn’t matter if I was using steel lines with a 45° double flare or stainless steel lines with a 37° single flare.


A before and after image of flared tubing

The Case for Lapping Flared Hard Lines

Lapping is a tried and true practice that goes back 100 years. Even with the best equipment, mechanics always lapped engine valves to ensure they wouldn’t leak. It occurred to me that lapping the flare on the brake line could get the tube to seat with the fitting.  I decided to apply this principle to flared tubing, and it fixed the leak I was getting in my Packard. This was how I came up with the idea for the Surseat Flare Lapping Tool.

How to Use the Surseat Flare Lapping Tool

1. Unscrew the brake line from the fitting.
2. Clamp the appropriate collet around the line.
3. Connect it to the Surseat and secure it with the lock spring.
4. Spray some lubricant to the lapping head before using it.
5. Rotate the lapping head back and forth about 6-10 times.
6. Clean and inspect the flare. There should be a smooth, concentric finish. If not, repeat step #5.
7. Remove the brake line from the Surseat and reconnect it to the fitting.
8. Bleed the brakes and check the seal.

How to Check for a Leak Where the Flare Joins the Fitting

If you have your brake or fuel line out of your vehicle, there’s an easy way to check to see if there’s a leak at the fitting. First, plug the end of the tube that you are checking by screwing in a cap flared tube nut or anything similar that will seal off that end of the tube. You can also purchase Earl’s Performance Pressure Test Kits if you don’t have anything that will work. Next, submerge the line in water and run air through the opposite end of the tube. If bubbles appear at the fitting, the seal is no good. Repeat the lapping process until you get no bubbles.

The bubbles on the left indicate a leaky brake line fitting
On the left you can see the bubbles coming out of the fitting indicating a bad seal between the flared tube and fitting. The bubble-free fitting on the right indicates a good seal.

Damaged Fittings

If a leak persists after lapping the flare with the Surseat, inspect the conical surface of the fitting for imperfections. Most of the time, the issue of sealing hard lines is with the flare, but there could be a problem with the fitting. A fitting can get damaged by over tightening them with an imperfect flare, by dropping something on them, or some other accident where you end up with a ding. Although it is not that common of an occurrence, it can be very problematic. For instance, what if you damage a fitting that’s welded into a tank?


Fitting with score in the conical surface
A leak in the fitting could be the result of a score in the conical surface of the fitting.

What People are Saying

“I have been double flaring brake lines for street rods for 30 years with great success. My latest project, a 32 Ford Roadster had 6 fittings that I could not get to seal. I kept trying to tighten the fittings, hoping the joint would seal. I don’t know if it’s the quality of brake lines or just me, but I couldn’t get the brake system to not weep. I remember seeing this tool at the Street Rods Nationals, and decided to order one, and see if it would solve my problems. I disassembled the entire brake system and lapped every flare with the Koul Tool P45. I reassembled the brake system and didn’t have a single issue. I will lap every fitting I make from now on just to make sure I get a proper seal on my brake fittings. The tool is easy to use and certainly performs.”

– Bill Kantos

“I’ve used several of your tools before, and have found all of them to be real time savers!
I had a master cylinder to brake line leak with my ’55 Chevy that your line lapper resolved. It seems that no matter how carefully I flare the tube, sometimes I still have a leak. It seems that the cause of this is roughness of the flare. Your lapping tool removes this making for a positive, leak free seal. Thanks again for saving me time, and trouble!”

– Ed Coyer

More About the Surseat

The Surseat flare lapping tools have a precision, diamond dust coated lapping head that will clean up tubing flares to enable them to seat with the brake line fitting. They come in three different models: The P-51 has interchangeable 37° and 45° lapping cones and will do 3/16 to 1/2 inch tubing. The P-45 and P-37 models are designed specifically for brake lines. The smaller size and rectangular shape make it easier to work in tight spaces inside the car. The P-45 and P-37, also known as ‘Minis’, are great for fixing a leak in an existing brake or fuel line, but they can also be used to lap-in your lines when installing them in order to avoid getting a leak in the first place.


Brake line sealing tool


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