(adapted from an article that I don't remember where I got it from!)

FINE PRINT: The information presented here is based upon First Amendment rights to say whatever silly thing one wants to. Anything that you might do, or any actions that might result from using this information is your own blooming fault. There is no way that this information is guaranteed to be 100% correct anyhow.

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 that 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 loadings 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". 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. 

Bundy brake pipes are available in various lengths at most parts stores, though they almost always have the wrong swivel nuts (no matter which one you need!). If you're doing a lot of this work, you can buy the Bundy tubing in 20 foot coils from Aircraft Spruce, Pegasus, or your local circle track store for about $8. 

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; various British parts suppliers can sell you new Girling parts, and aircraft and race suppliers will happily take your money for AN fittings. 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 (more or less) flaring styles in common use for brake systems. British cars have a bubble flare (a.k.a. Girling 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 (a.k.a. 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; see below). 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 bullet-proofness 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, once something has been put on the car, it always needs to come off at some point (forgot , or broke , or need to swap out  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. 

There are 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. 

Most British cars 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. Thus it's possible to replace 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. Most master cylinders all have machined flat surfaces, so the outlets are easy to adapt to -3 hoses as well (-4 for the clutch, typically). 

A few words about Teflon lines: they may not have a place on your street car. For starters, the Teflon lines you make up at home don't have DOT approval, because they haven't been through the DOT approval process and you're not a certified manufacturer. Even the Teflon lines you can buy pre-built are probably not DOT approved (though there are a few coming onto the market) for the same certification reasons. The main reason for this lack of approval is that manufacturers of racing gear don't care to spend the money - they're building the hoses for racers. The next reason is that the DOT expected lifetime for such brake hoses is five years, and the prudent racer will replace his or her brake hoses every other season, just to be sure. (This is a good reason to make your own - then you can reuse the fittings and just buy new olives and hose.) There are some concerns about using Teflon line in cold environments, though Aircraft Spruce is willing to sell you lines custom made from Aeroquip Teflon hose and rate them to -40 degF, so it may not be a concern. It may well be that the intended application for those lines is not subject to as much repeated deflection as the flex line snaking through your front suspension out to the caliper. 

Other sealing methods

There is one other kind of sealing involved in braking systems, usually at caliper or cylinder entry and often when converting over from one kind of flaring system to another: a washer face at the top of a hole. This requires a crush washer of soft copper, aluminum, or in extreme cases, a Stat-O-Seal, which is a combination of O-ring and aluminum crush washer. To make this an effective seal, a precision machined surface is required around the drilling, exactly perpendicular to the axis of the threaded hole. If this is not there, you must use a bottom sealing fitting.  Conversely, if the hole does not have a nicely formed drill point form at its bottom of the fitting will not reach the bottom, you must use a sealing washer (and, of course, provide the machined washer face for it to seal against). 

A neat trick is that copper crush washers can be reused - IF you anneal them. Place the washer in question on a brick and play your propane torch across it until it glows orange. Let it cool. It's now as soft as it was originally and will re-crush and seal perfectly. 

Vendors

Good flaring tools are easy to come by. Get the catalog from: 

Aircraft Spruce and Specialty Company 
201 W. Truslow Avenue 
P.O. Box 424 
Fullerton, CA 92632 
(800) 824-1930 order hotline 
(714) 870-7551 
(714) 870-7315 customer service 
(714) 871-7289 fax 

They'll charge you $5 for the catalog, spend almost that much shipping it, and include a gift certificate for $5 on your first order. Well worth the trouble. This company specializes in selling stuff to aircraft homebuilders - that makes it an excellent source for weird tools, small quantities of raw materials (ever try to buy just 2 feet of 4130 chrome-moly tubing before?), GOOD hardware (meaning NAS and AN mil/aero-spec fasteners) and other nifty arcane thinguses. If you are in a part of the country that doesn't have places that sell to us crazed enthusiast-types, you need this catalog. 

They have the Rol-Aire Imperial, which is the flaring tool of choice for 37deg AN single flares, and several SAE and ISO flaring tools as well. Another source of interesting tools for the home mechanic is Eastwood. 

Eastwood Company 
580 Lancaster Avenue 
Box 296 
Malvern, PA 19355 
(800) 345-1178 

Dave Bean Engineering should be on the vendor list of anyone dealing with a car that was built in England: 

Dave Bean Engineering, Inc. 
636 E. St. Charles Street, Star Route 3 
San Andreas, CA 95249-9564 
(209) 754-5802