The brake lines were talking about are the flexible ones that connect between the
hard lines (i.e., the inflexible tubing) in the car and the brake calipers on the wheels.
Theyve traditionally been made from rubber tubing, with steel or aluminum
connectors crimped onto their ends. Nearly all passenger cars are shipped with rubber
brake lines, and they hardly ever fail.
"Stainless-steel" lines are made of Teflon tubing, not rubber. Teflon has a
number of advantages over rubber; the chief ones are that it doesnt expand under
pressure and it doesnt deteriorate with age. It also resists high temperatures and
is chemically inert, so its compatible with all brake fluids.
However, Teflon is pretty fragile, so it has to be protected from physical damage
(chafing, flying rocks, etc.). Although some manufacturers armor their Teflon hoses with
Kevlar, most protect the Teflon with an external sheath of braided stainless-steel wire...
So thats why armored Teflon hose is usually called "stainless-steel hose".
The ends of the hoses have to be securely attached to the brake calipers and the hard
lines, so each hose is terminated by threaded hose-ends.
Those hose-end fittings can be attached to the hoses a couple of ways.
The cheap way is to crimp or swage them onto the hoses, like the fittings on rubber
hoses. The more-expensive way is to use a two-piece replaceable hose end that captures a
portion of the hose between an inner nipple and a concentric outer socket. These hose-ends
(often referred to generically as "Aeroquip fittings" because they were invented
by the Aeroquip Corporation) are used EVERYWHERE on aircraft and race cars.
First, I should point out that there may be lines available that meet
all the DOT specs, but are non-approved only because they havent been submitted to
the DOT for approval.
Manufacturers cant legally say that their lines are approved - even if they KNOW
that the lines meet all the DOT specifications - without actually submitting them to the
For that reason, stainless-steel brake lines can fall into three categories:
"DOT approved" - These lines have been submitted to and approved by the US
Department of Transportation.
"non-approved" - These lines dont have a DOT approval, either because
they dont meet the specs or simply because they havent been submitted for
"non-conforming" - These lines are non-approved (and non- approvable) because
they fail to meet the DOT specs.
The safety standard that brake hoses must meet is called Federal Motor Vehicle Safety
Standard 106; if you have a copy of the Code of Federal Regulations handy, its in
Title 49, Volume 5, Subpart B, Section 571.106.
The section that applies to hydraulic hoses is about six pages long, and it covers
everything from labeling requirements to pressure and temperature testing.
One important thing to note -thisll come up later when I explain why the
"best" hose assemblies cant be DOT approved- is that each of the
requirements in the Standard carries the same weight; if a hose fails to meet ANY
requirement, it wont be approved.
Hypothetically, therefore, a hose which met all the performance specs but was labeled
in lowercase letters (the Standard requires block capitals) would fail to be approved.
Also, some of the features required by the Standard provide a certain amount of
"idiot-proofing", but at the expense of absolute maximum strength or safety...
Its the same sort of mandated mediocrity that forced Ferrari to replace the stock
5-point safety harnesses in US-spec F40s with those ridiculous motorized-mouse single
Most of the "performance" specs in the Standard (i.e., burst strength,
compatibility with brake fluids, tensile strength, expansion under pressure, etc.) are
easily met by all halfway-decent hydraulic brake hoses, but there are a couple of tests
and requirements that are particularly difficult for stainless-steel hoses to meet.
Those requirements are:
1. The manner in which the fittings must be attached to the hose.FMVSS 106 specifies
that "Each hydraulic brake hose assembly shall have PERMANENTLY ATTACHED brake hose
end fittings which are attached by deformation of the fitting about the hose BY CRIMPING
OR SWAGING." [Emphasis added]
The idea is that, since crimped-on fittings cant be loosened, a stupid end-user
wont be able to screw with and weaken them.
This is a good thing from a product-liability point of view, I guess... But it means
that any hose assembly which uses the very best fittings available -like the
nipple-and-cutter Aeroquip Super Gem or Earls Speed Seal- is non-conforming and
CANT be DOT-approved.
2. The "whip-resistance" test. This test involves mounting the hose on a
flexing machine, pressurizing it to 235 psi, then running it at 800 RPM for 35 hours.
When steel-armored hoses were run through that test, it was found that the hoses tended
to bend right at the junction between the hose and the hose-ends. After a while, the
stainless-steel braid would start to tear, and the broken wires would cut into the inner
Teflon liner, causing it to fail.
One brake-hose manufacturer fought to modify the whip test, claiming that their
stainless-steel hose could easily comply with the test if only a supplemental support were
used during testing to move the flexing-point away from the hose-ends.
The NHTSA ruled on the issue in August, 1996, deciding to allow manufacturers to use
the supplemental support... But only on the condition that the same support was used when
the hoses were installed on a real car.
FMVSS 106 was modified to include the use of the support, and the new rules went into
effect in October, 1996.
"DOT-approved" stainless-steel brake hoses went on sale immediately
So now you know what DOT approval entails. For information on why you would want (or
might not want) stainless-steel brake hoses on your car, see read on...
There are three reasons to install stainless-steel brake lines:
1. They look racy.
2. They dont swell like rubber lines, so they can potentially firm up your brake
3. If youre doing a lot of off-road driving, the stainless-steel braid may
protect your lines from being punctured by rocks or whatever.
Heres the thing, though: Since stainless-steel lines dont bulge as they
age, and since the inner Teflon lining is hidden behind the braid, theres no easy
way to inspect the lines for warning signs of imminent failure.
This is no big deal on a race car, since the lines are (or should be) replaced at least
once a season. On a street car, where most people are likely to let YEARS go by without
even looking at their lines, it can be an issue.
Plenty of people, therefore, warn that you should use rubber lines instead of stainless
steel... Theyll be happy to give you anecdotal evidence of steel lines simply
bursting (or, more commonly, separating from their hose-ends) catastrophically and with no
I havent seen any references to this sort of failure that mentioned whether the
REAL Aeroquip or Earls hose, attached properly to the CORRECT hose-ends, and
installed properly on the car.
I define "correct hose-ends" as Earls Speed-Seal (the new name for
Fluor-O-Seal) or Aeroquip Super Gem, and "real hose" as Earls Speed-Flex
(the new name for Fluor-O-Flex) or Aeroquip... uhh... I forget their brand name.
Anyway, Speed-Seal hose-ends work just like Earls Swivel-Seal ends; the hose-end
can swivel after assembly. The nipple/cutter assembly on these ends (and on Aeroquip Super
Gem ends) was specifically developed to prevent blowoff of the hose-end... Im still
waiting to hear from anyone who has firsthand knowledge of one of these hose assemblies
coming apart, and until I hear from that person, I run "real" stainless-steel
lines on my car and replace them regularly.
THIS IS IMPORTANT: The lines that your performance-parts distributor will sell you are
made with no-name hose from God-knows-where (probably Taiwan), and the hose-ends are just
swaged-on fittings that are an invitation for disaster. I wont put these on my car,
and I dont recommend that you put them on yours, either.
There are now "DOT-approved" stainless-steel lines. I have no idea what they
are, but I suspect that they STILL use cheap-ass crimped-on hose ends. Until my suspicions
are disproven, I wont put THEM on my car, either.
If you do decide to put stainless-steel lines on your car, you need to be aware of a
1. When you install them, you must make SURE that they cant kink, twist, or
stretch under any combination of wheel droop, bump, or (for the front wheels) steer.
2. The stainless-steel outer braid will cut through anything against which it rubs, so
you have to make sure that the lines dont rub back and forth over anything
3. Stainless steel lines have been known to fail when dirt gets between the outer braid
and the Teflon lining... As the braid moves back and forth, the dirt abrades the Teflon
and can make it rupture. If you look at stainless-steel lines on motorcycles, youll
see that many of them are encased in plastic tubing, apparently in an effort to eliminate
this problem. The tubing also helps considerably with the abrasion issue mentioned above.
Andrew Warren - firstname.lastname@example.org