Stainless Steel Braided Brake Lines
I bought stainless steel braided brake lines for my '78, and when they arrived, they have tags on them saying not for strret use. Is this because they are not safe or because they are not DOT approved.
I don't remember the supplier having a disclaimer for "off road use."
78 5 speed
Stainless steel lines typically have a teflon inner liner. Teflon is wonderful stuff, and does not expand as much as a similar rubber hose would, thereby (supposedly) giving better pedal feel. Problem is, teflon tends to take a "cold set" after a bit, and any bending movement beyond a certain limited degree will cause the inner liner to kink. That would be a bad thing!
On a race car where items like this are regularly replaced, it is no big deal. On street cars where we typically install it and forget it, they are not recommended.
86.5 Indischrot 5-speed
I have never seen a factory line fail yet I've seen many stainless lines fail but probably due to abuse like letting the caliper hang from them.
This has come up before. The deal is that Teflon does not have any rebound (or memory). Why that's important is not in the short term but in life expectancy. A brake system is a finicky container that likes to keep a small amount of fluid in the right places for use when needed. There are many potential leaks in the system, 8 of which are the crimped ends of the brake hoses at each caliper. Each end fitting is basically a hose barb that the flexible hose is tightly slipped over and then an outer crimp sleeve is crushed in place to make a permanent connection. Nitrile rubber (Buna N) is a normal material for the hose using regular brake fluid (basically mineral oil). This soft rubber will remain in place for 10 or more years without drying out and loosening in the crimped area of the hose. Teflon on the other hand just compresses and then loosens immediately. It might last a year or it might last a week or maybe only one race. Creep is a term used to describe what happens to Teflon as it moves under compressive loads.
The reason people use Teflon is more temperature than anything else. Where is that important? On the track. What else is true about the track? Most people don't care about their cheap parts lasting for next season. What else is true about Teflon? To have the same tensile strength as a rubber it is more of a rigid plastic. What does that mean? It doesn't give as much under internal pressure. What does that mean? It gives a slightly stiffer pedal at the limits of use. Why is the stainless braid on a hose assembly like a teflon hose? Cause it can't take repeated pulls from wheel movement. It doesn't back it up for pressure reasons only pull out at the crimped fitting.
Why should you think my opinion is worth anything? Well, my company buys about 3 million dollars worth of Teflon products from Dupont and W.L. Gore (GoreTex) every year. I spend alot of time discussing sealing characteristics of these products with their engineers.
Send the Teflon lines back and get OEM ones. Sleep better tonight.
79 US 5 speed WIP
Jay, I agree with your judgement of the advisability of using the stainless braid over teflon brake lines. However, I would just like to set straight the one thing. It's about how these hoses seal the teflon at the end fittings. I have a couple of these lines disassembled on my workbench at this moment. I used them to make new fuel distributing hoses for the CIS fuel injection system on my old SAAB. Got them from Earl's.
The thing is that they do not use a hose barb inside the teflon, with a crushing sleeve down around the outside like is normally done with rubber lines. And they don't precisely because of what you said about the cold-flow properties of teflon. What they do is capture the teflon with the stainless braid and treat it like the fragile "balloon" that it is. It is the braid that they crimp down on from both inside and outside with barbed sleeves. When I first took one apart, I couldn't believe it! The biggest danger would be for the teflon to be forced off its little thin smooth inner sealing sleeve by the axial hydraulic pressure applied. But what keeps it trapped is that the braid over the outside is restrained axially, and then the teflon can't go anywhere because there is a fitting at the other end. When I finally started thinking about the line as a system having two ends, I began to understand how this sealing system works.
But whatever I am saying here does not disagree with your original opinion that the stainless braid-over-teflon is not as long-term reliable as the standard rubber-molded with fabric.
BUT! What would we have if we used the stainless braid over a rubber hose? It could give us the harder pedal feel without being as fatigue prone as the teflon.
It is true that there are more than one style of trapping the Teflon on the end of the hose. I didn't want to go into those nuances but you are absolutely correct. Regardless, there is a junction in every design between the fixed steel end fitting and the flexible hose. In order to contain pressure somewhere you have to generate the same amount of compression between the hose material and the end fitting. If you need a proof pressure of say 75psi then you need to gaurantee that the compression exceeds the 75 psi at the juncture or it will leak when brand new even. Compression will creep Teflon materials with a fixed and low number of cycles. There is no known design that eliminates the effect of creep in Teflon assemblies short of bonding the Teflon hose to the tube. There is only one known adhesive that is capable of bonding Teflon to itself or anything else and that is not available to anyone but people in the R&D trial stream (like me) and it still has temperature issues. W.L. Gore of GoreTex fame is working on ways to solve these problems for industry and they have some pretty far out ideas that I have been privy to. Confidentiality agreements keep me from sharing but I will say that A LOT of money is being spent on this problem. Hopefully some day there will be a breakthrough in this field.
The hose style you describe will work perfect on your SAAB. That style of CIS like on my Audi has the FD mounted on the fender liner with the injector lines travelling a ways to the injectors. These are only slight vibrations over a relatively long hose. This is nothing like the pounding that a brake line is subjected to. However, that being said a CIS injection system has some pressure spikes over 75psi during normal operation and that is an issue. Cycles will generate leaks over time period so they are not going to be as reliable as properly configured steel ones.
One more thing about the stainless steel braiding. This is not to give the hose a higher pressure rating at all, period. Remember the chinese finger trap we all played with as kids. They conform to any size fingers and allowed some twisting but they wouldn't allow you to PULL your fingers out. That's what the steel braid does. If it were to contain pressure it would be annular or a tight helical wrap not a 45° or 60° braid. It is there because the Teflon hose is not as strong as the rubber hose and pull cycles fatigue the connection. Call any hose manufacturer and talk to their engineering department. Braid will not help a rubber brake line have better feel. Bigger calipers will though.
The reason that most stainless lines do not pass DOT testing, is due to the fatigue resistance of the stainless wire used in the braid itself is extremely low. The most common failure point in stainless lines is in the high stress area around the end fittings where there is not sufficient strain relief.
Some companies who offer stainless lines have adapted a "spring" type strain relief at the junction of the fitting and line, and they have passed DOT.
Will they stretch less under high braking force...yes. Are they suitable for racing...yes, if they are checked every race and replaced on schedule. Are they suitable for street use where they are to be installed and never looked at again or never replaced at regular intervals....no.
Jim Murphy is currently testing some stainless, DOT approved lines, they have been on his 928 for almost 2 years now and we will review the condition of the braid in a few months and see how it has held up against daily usage.
Best bet....stick to the factory lines and never worry for 10 years or so of any kind of use!
Marc M. Thomas
928 Tips Home Greg's Home