HOME RESTORATION.
Everything You Wanted to Know About Brake Plumbing.
This article is a sincere effort to impart a basic understanding of the complex arena of brake plumbing. If, while attempting to apply any of the ideas, procedures or suggestions herein, you should experience a brake system failure, it will be as a result of your own conscious decision. The author disclaims responsibility for your actions - and your accident.
English cars are almost always plumbed in 3/16" tubing, which utilizes standard 3/8-24 UNF thread swivel nuts. The exception is the piping used around tandem master cylinder differential valves, which sometimes use 7/16-20 UNF swivel nuts in certain circuits to prevent incorrect routing or cross plumbing. The other exceptions are certain clutch hoses which require the high-flow capacity afforded by the 1/4" line (hence the 7/16-20 UNF thread).
Brake lines should always be fabricated from Bundy tubing having a wall thickness of 0.028" minimum. Brake line pressures can and do exceed 1000 psi. On no account should copper, aluminum, or commercial fuel line be used, despite the fact that they are easily available and appear to be easy to work with. Pure copper tubing work-hardens and becomes very, very brittle- especially at flare fittings. Under the cyclical loading seen from brake applications, and in the presence of moisture (and possibly high concentrations of chlorides, if you live in a road salt area), they will almost certainly fail. They will tend to crack and fail right at the flare, generally with little or no warning. (Copper also catalyzes the auto-oxidation of fuel, the mechanism of sludge formation. Copper should never be in contact with fuel.) The British have developed what they call "copper brake pipe", which seems to be a seamless tube made of a copper alloy that bends easily, does not rust, and is in reliable use for "classic cars". I don't have experience with this stuff, so I won't say good or bad about it - I do know that it is different from pure copper. It is almost certainly not as reliable as honest-to-gosh Bundy tubing, and thus shouldn't be used on a race car.
The swivel nuts from the old lines can sometimes be re-used if they are in good shape. But chances are that you're reading this because you want to upgrade your fittings, rather than just replacing what's there. Most auto parts stores can supply the standard fittings that are on current production cars. Play it safe - if you're in doubt, buy new. While you're at it, you might follow the lead of many folks doing race car prep and only use steel fittings, even though aluminum is available and a bit less expensive.
Know your flares.
There are four flaring styles in common use for brake systems. British cars have a bubble flare backed up with a male swivel nut or a 45 degree double flare backed up with a female swivel nut. Metric cars have ISO bubble flares, where the pipes and threads are metric sizes rather than inch. Detroit iron has a 45 degree double flare backed up by a male threaded nut. Most (non-British) race cars are plumbed with AN (aka JIC) type single flares - a 37 degree single flare with a backup sleeve and inch threaded swivel nut (some people make a double flare here, which is useless overkill and may lead to failure). Lastly, some brake fittings use tapered pipe fittings.
Take the easy one first: tapered pipe fittings are not really a positive seal under adverse conditions. They may do the job for a street car, but they certainly have no place on a race car.
The bubble flare is used with a male swivel nut, and seals at the bottom of a drilled and tapped hole, with a nice angled bottom. While it can usually be resealed, it has a limited lifetime - there's no good way to get back the deformation that was crushed out for the first seal, short of remaking the flare from scratch.
Making such a flare is easy: if you are in possession of a standard 45 degree double flaring tool, the bubble flare is what results after the first half of the operation.
Simply stop there, and you have the bubble flare which will seat nicely at the bottom of the hole. If you continue, inverting the form tool and finishing the job, you then have the more familiar double flare used by Girling and the US automotive industry.
The SAE 45 degree double flare usually has a male-threaded tube nut that bears directly on the OD of the flared tube- so you need a double flare to help control galling that can result in stress cracking right at the flare. In short, you need "give" there. Problem is, the deformation that results is kind of irreversible, so the next cycle or two will result in your having to use astronomical torques to keep the flare from weeping. Worse is trying to use a single flare in an SAE flare nut and seat, and worse still if the seat is brass- the flared tube is squashed from both sides, even as it is deformed by the nut galling on it. The brass seat deforms and work hardens. It may seal once, with a ton of torque and some luck. It's not recommended practice - it's not even a good idea.
Racers (and aircraft, which is where the system originated as the "Army-Navy" or AN standard in WWII) use the single 37 degree flare. The AN single flare is still a concave flare, but its 37deg angle seals by stretching, not squashing. The tube is supported by a separate sleeve that the female-threaded tube nut bears upon. This isolates the flare from the torques imparted by the nut. So rather than trying to get a seal despite the presence of rotating torques and the resulting galling, you press the flare between precisely-machined (steel!) seat and precisely-machined support sleeve. The sealing area under compression is at least double that of the SAE flare. An additional bonus is that the OD of the nut is a lot larger than the 3/8" of an SAE nut, which means you won't kill as many trying to get the proper sealing torque. (Even so, you should always use a proper flare nut wrench on any tube nut.)
The SAE stuff was designed to go together once on the assembly line, and then be "immortal", as defined by Detroit. It's pretty good at it, too! The AN stuff is designed for field serviceability, long fatigue life, and a level of bulletproofness the SAE never considered. A further, Very Strong, recommendation is that single flared AN fittings are the only thing that Carroll Smith will suffer to put on his race cars.
The reason for harping on repeatability and multiple mate-demate cycles is that, to the best of my knowledge, I have never once put something on the car, and had it stay put on. I always forgot something, or broke something during the season, or needed to swap out something because it was at the end of its service life. While the double flare has that nice "squish" feeling as you tighten the flare nut the first time, the AN fitting has the same torque requirement for the second mating cycle as the first. That's where the reliability across multiple cycles comes from.
Never use compression fittings for brake plumbing.
I know racers that have gone so far as to purge all the SAE double flare fittings out of the plumbing of their track cars, except possibly the fittings at the master cylinder. The rationale is two-fold: first, the above-mentioned repeatability, and second, the fact that the AN parts are universally available at every race shop in every backwater in the country.
I have not gone quite so far on my British cars, which are plumbed with Girling flares. It is a happy coincidence (at least, it seems like a coincidence, perhaps it is by design) that the female Girling flare hardware will mate comfortably with a male AN-3 fitting; I have thus replaced the fancy fittings on the rubber hoses with the appropriate AN-3 male-to-male bulkhead fittings, and the hoses themselves with -3 female fittings on Teflon -3 line. My master cylinders all have machined flat surfaces, so the outlets are easy to adapt to -3 hoses as well (-4 for the clutch, typically).
Good Luck.
Part 2 next month.
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January 2001