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Racing Technology
The right way to plumb brake flex hoses
by Wayne Scraba
Here's a scary thought: You stand on the brake pedal of your race car at the end of a quick pass down the 1,320. The fluid pressure exceeds 1,000psi. Then the pedal goes to the floor. I won't explain what happens next (let your imagination be your guide), but I do know the cause of the grief was likely a fitting that was blown off the end of a braided brake hose.
Sound impossible? Not so. I've visited plenty of racers feverishly working on their cars in their own shops, and one of the most perplexing situations I've seen is someone building AN (braided stainless-steel-reinforced) brake hoses. More often than not, the hose is assembled incorrectly. The sad truth is, it's not hard to do. When planning and building brake flex hoses for a race car application, you have but a few choices: OEM rubber flex hose, home-assembled -3 AN Teflon core hose, or pre-assembled -3 AN Teflon core hose.
Considerations
Before going further, some facts about hose must be considered. You cannot use neoprene-lined AN hose for brakes. Under brake-system pressure, a neoprene-lined hose will expand, which in turn soaks up brake-pedal travel. Instead of forcing the brake piston inside the caliper against the brake rotor, the hydraulic forces take the path of least resistance and expand the hose. In the mid-1960s, Earl's Performance Products pioneered the use of armored flex hose with an extruded Teflon core to solve the problem. The stiffness of the Teflon liner combined with the tightly woven, high-tensile stainless-steel outer braid fixed the brake-pedal-travel dilemma and offered another advantage: By eliminating hose swell, which is still present in OEM brake hoses to some degree, pedal firmness and feel are improved significantly. Of course, the stainless-steel outer braid improves the abrasion resistance, and the design of this hose increases temperature capacity as well.
Before you run out and buy some
bargain-basement Teflon hose, there's one catch: Two types of Teflon-lined, braided stainless-steel hose are available, commercial specification and aircraft specification. The difference is in the wall thicknesses of the Teflon. Commercial hose has a 0.030-inch thickness, and the aircraft-specification hose has a 0.040-inch thickness. Further, some hose is manufactured with a stainless braid that is loose on the Teflon liner. This type of hose offers good pressure capacity, but it's limited in its resistance to expansion. This article will discuss only aircraft-specification hose that is bonded tightly to the stainless-steel braid.
Tedious operation
Given the benefits of the -3 AN hose for brake applications, it seems the way to go. It is — unless you mess up the hose-end installation. Hose-end installation is not complicated and no special tools are required, but improper installation can result in a catastrophe of the worst kind.
During assembly, the dilemma usually occurs when installing the sleeve, sometimes called an "olive," between the stainless-steel outer braid and the Teflon core. A small tool is available for this job (Earl's sells one under part number 007), but a small screwdriver or scribe can be used. Unfortunately, separating the braid from the liner is more difficult than it sounds. This can sometimes become a tedious job, and you have to be careful not to mark the Teflon. You also have to be absolutely positive that none of the braid is trapped between the Teflon core and the sleeve. When you've reached that point, you then have to be sure that the Teflon core is completely bottomed against the inside of the sleeve. Finally, you must be sure that the sleeve is square in relation to the Teflon core.
Part and parcel of the operation is usually a succession of holes punctured in your fingertips. The stainless-steel outer braid is sharp, and the more you handle the cut hose, the better the chances of wounding yourself. Additionally, if you don't follow the instructions carefully, there's a good chance that the hose end will back out under pressure.
Some racers become frustrated with the process and the bleeding fingers that go with it. This is the point where I've made my observations about brake-hose assembly: Most of the time, the "that's good enough" attitude prevails, even if the internal sleeve isn't installed correctly. Then the hose is assembled, and Lady Luck is your only ally. Evidence of an improperly assembled hose end on -3 Teflon hose is the final gap between the face of the socket and the hex of the nipple. According to Earl's, it should be between 0.023- and 0.046-inch, which must be checked using a feeler gauge. I've seen plenty of assemblies that exceed these dimensions by a significant margin.
Alternative service
Is there another option besides painstaking assembly? You bet. It's called a pre-assembled, crimped Teflon brake hose, and Aeroquip, Earl's, Russell, XRP, and others have offered it for years. With this type of racing brake hose, the hose ends are installed by one of two methods: The end is either swaged in place or crimped. In either case, the hose-end attachment is permanent. Once the manufacturer attaches the ends, each hose is pressure-checked. Typically, a hydrostatic pressure test is used that goes to 4,000psi on each hose. Obviously, this far exceeds the pressures of a brake system, and the home-based race mechanic has no way to test the hose to this pressure.
What types of hose-end combinations are available? The accompanying photos show three common styles, but each manufacturer offers racing brake-hose packages in countless configurations. You can specify -3 straight female swivel ends, -3 90-degree female swivel ends, 10mm-3/8-inch banjo (.425-inch-thick) ends, 7/16-inch banjo (.425-inch-thick) ends, 3/16-inch tube inverted flare (3/8-24 thread) female ends, 3/16-inch inverted flare (3/8-24) male ends, 10mmx1 female ends, 10 mmx1 male ends, and several -4 AN configurations for pressure-gauge and hydraulic-clutch applications.
That's not all. You can mix and match each of these end configurations, and in each hose configuration, you can specify the length. For example, I used a -3 AN female swivel to a 10mm banjo on one side of the Firebird in the photos. In this particular combination, 20 lengths were available, ranging from 13 to 48 inches. If that's not enough, the manufacturers can build a custom hose in any configuration and length. Bottom line? Any type of brake-hose end and length is readily available.
I should emphasize that the hoses mentioned in this article are for racing purposes only. For example, Earl's has independent test lab reports that show that the hoses exceed DOT recommendations in all categories, but it isn't making claims that the hoses are DOT certified. After installation, Earl's recommends that each hose be kept clean and free-flowing. There must not be any possibility that the installed hose assemblies can stretch, crimp, or kink under any conditions of wheel travel and steering angle. Earl's also urges periodic inspection of all brake hoses for condition; this is a good idea for any form of brake hose.
Is the service cost-efficient? Absolutely. Basically, the costs are close to the price of the bare hose and basic fittings and a few bandages, which you'll definitely need if you make up your own hose. Regarding delivery, the hose in the photos arrived a few days after ordering by UPS ground. The best part is you'll gain some peace of mind. That might be more important than you think when it comes to brakes.