Fuel & Brake Line Lapping Tool
Channel: Classic Car Restoration Club
Date: 20/04/2023

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We’ve all been there: you just bent up a new brake line and installed it, and as soon as you start to bleed the system you notice a drip at the flare nut fitting. Of course the first step is often to tighten it some more and sometimes that stops it, but other times it continues to drip. So you grab the wrench again and tighten it beyond where you are comfortable tightening it to, only to have it continue to drip. Eventually you either round off the flare nut fitting, damage the threads on the flare nut seat or just give up and cut off the tubing flare and re-flare the connection.

The problem is actually in the flaring process itself. When creating an inverted flare on a brake line the metal is stretched, shrunk and inverted and occasionally ridges develop in the flared fitting. Small deformations in the fitting will smooth out when the fitting is tightened into the seat but sometimes these ridges are too large or the material is too hard to smooth out during normal tightening. This can be especially true when bending stainless brake lines.

One great solution to this problem is brought to us by the folks at Koul Tools who manufacture a great tool for lapping the ends of the flare fitting to create a smooth positive seal without the need to over-tighten your fittings. We join Mark Simpson in the shop to test out the Koul Tools P-51B Surseat Flare Lapping Tool. This tool allows you to lap-in flared tubing to eliminate leaks in fuel and brake line joints.

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This tool works by removing irregularities in the flare and hones it to a perfect angle to enable the tube to seat with the fitting. It’s not only perfect for your newly-flared stainless steel, aluminum or copper lines, it can also repair the seal with the fitting of an existing brake or fuel line. It is based on the tried and true practice of lapping engine valves. A diamond dust coated lapping head with a precision 37° or 45° angle literally polishes the flare smooth to perfection. But why wait to discover a leak when bleeding your brakes to use this tool when it makes great sense to use it every time you make a flare fitting to ensure a great seal.