"They got two stops, and then no brakes"

I wanted to spend a post talking about the hydraulic connections in your clutch, brakes, and fuel because I have had a really hard time getting educated on this either through books and through the web.

Overview

The fluid in the brake system, the clutch and the fuel system is under relatively high pressure (~1500psi for brakes) and the system that conveys it from your pedal to the brake calipers or clutch slave, must hold that pressure without significant flex, support relative motion of the suspension, and be completely reliable. Therefore things like hose clamps and radiator hoses won’t cut it.

Lines and Hoses

Because your brakes multiply the force from your foot many times, the motion of the brake caliper piston is very slight. Because there is so little motion, very little fluid moves when you step on the brake. If the hoses or lines expanded much, an air or moisture bubble got in there, or you sprung a significant leak, then one full pedal stroke wouldn’t build enough pressure to stop the car.

Since almost no fluid is moving, the lines can be very small. In your car as delivered from the factory most of these lines are rigid tubing that which is all tucked into places safe from heat and almost any source of mechanical damage. For brakes these lines are small 3/16 OD ~.032 wall thickness. For your clutch, where significant amounts of fluid do flow, (1/4 inch OD is used.)

When the line connects to something moving (like from the rigid chassis to the moving suspension), the rigid lines are connected to a flexible hose to avoid the rapid fatiguing of the rigid line.

Flare vs inverted flare

The connection of lines and hoses must be completely sealed under high pressure, and completely reliable. On your factory delivered car this is done with the inverted flare fitting. Here is where the whole thing pretty much blows up and you get very little help from the web. We are interested in cars, These typs of connections are used across the fluid handling industry, and there are many different variants that are only approximately standardized. I am going to try and describe here what is of interest to us and why.

Here is an inverted flare fitting:

Here is a flare fitting:

Please note that whether you are talking about the male piece or the female piece of the top image, you are talking about inverted flare. Flare fittings, as far as I can tell, are not of interest on cars.

Notice that the inverted flare nut in the top picture is captured by the line. Even if you dis-assemble the pieces, you cannot get the male nut off the tube. So when you make these lines, the nut is placed on the tube, then the tube end is flared. There are three common types of flares, (and many more less common). Shown below, left to right are, single flare, bubble flare, and double flare.

Bubble flare is used on all metric size tubing.

Double flare is used over single on small tubes, and especially with stronger materials like stainless steel to avoid splitting the flare from the large strain.

As far as I can tell, Single and double flare fittings are interchangeable. Bubble flare absolutely is not. You will find a bunch of stuff around saying that people have had good luck using double flare in bubble flare fittings and it seals when they don’t have access to bubble flare tools. Do not do that! This is your brake system. Darwin is at work here.

Since Darwin is in play, the quality of the forming tool matters. The dies on the cheaper tools don’t always hold the line securely and crimp evenly. Also, many of the cheaper tools don’t have enough power to deal with stainless. Some of the better tools I have found are made by Ktool, Eastwood, and Mastercool. Pay attention to what tube sizes and flare types come with your tool, and what even can be gotten separately.

SAE vs AN

Please note there is another type of fitting that looks just like the SAE fitting described above, and is used on cars, and is 100% not compatible with the fittings so far discussed. This is the AN fitting. The angle of the SAE fittings used on your car is 45°. On the AN system it is 37°. AN hardware connectors are common on aftermarket plumbing especially with fuel systems. This is what aerospace, and much of the aftermarket uses. As Carroll Smith points out design and assembly of these systems is “straightforward and expensive.” AN hardware is often recognizable by the red and blue connections, although those colors are not standardized. AN sizes are specified by dash number.

Metric vs SAE

As if there weren’t enough choices, the addition of metric adds more than one other choice for hard lines:

SAE tube, SAE threads

SAE tube, Metric threads

Metric tube, Metric threads

Also note that thread pitch is common with the metric stuff. For 6mm tube you can get inverted flare nuts in either M12 – 1.0, M12 – 1.5, or M14 – 1.5.

Lastly note that M10 – 1.0 and 3/8 – 24 are so close in size that they are interchangeable… except that M10 will be bubble flare, so they are not interchangeable and the shape of the cone in the bottom is how you can tell them apart.