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Brake Lines and Hoses
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As stated back in Chapter 3, the brake lines and brake hoses have one of the easiest jobs in the
brake system. They only need to transport pressurized brake fluid away from the master cylinder
and to the four corners of the car. On the surface, it sounds like a simple job.
However, while the brake lines and hoses have a relatively straightforward role to play in the brake
system, if they’re unable to perform their task, everything goes really bad, really fast. There are few
brake system failures that draw as much immediate attention as a hydraulic fluid leak. So while the
job at hand may be a simple one, the importance is no less significant than for any other brake
system component.
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Hydraulic Circuit Design
In Chapter 5 it was briefly stated that all modern vehicles employ two completely separate hydraulic
circuits to direct brake fluid pressure to the four corners of the vehicle. This step is taken in the
interest of safety—if one circuit fails and is unable to generate pressure, the other remains capable
of providing at least some level of brake system functionality.
Front-Rear Split
One of the simplest methods for achieving brake system redundancy is to attach the front brakes to
one dedicated hydraulic circuit while placing the rear brakes on their own separate hydraulic circuit.
Generally referred to as a front-rear split, this hydraulic architecture is found on vehicles that have
relatively uniform front-to-rear static weight distributions.
The primary performance advantage of the front-rear split design is that in the event of a failed
hydraulic circuit, there are still two brakes on the same axle that provide equal braking forces. For
this reason, the vehicle won’t turn or pull in either direction under failed-circuit braking.
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From a manufacturing perspective, a front-rear split design only requires that a single hydraulic line
be routed from the master cylinder to the rear of the vehicle. Once the hydraulic line reaches the
rear axle it can be split to provide pressure to both the left rear and right rear brakes, but a single
hydraulic line can be used for a majority of the distance.
One potential disadvantage of the front-rear split is that the level of deceleration available under
failed-circuit braking varies with which axle experiences a hydraulic failure. In the case of a failed
rear hydraulic circuit, the driver retains braking on the front axle, but a failed front circuit only
leaves the driver with the rear brakes to slow the vehicle. Because the gain of the front brakes is
generally much greater than the gain of the rear brakes, this can result in a significant variation in
performance between the two failed conditions.
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In a front-rear split design, the two
front brakes (solid line) and two rear
brakes (dashed line) use independent
hydraulic circuits. This architecture is
most common in rear-wheel drive
vehicles and/or pickup trucks due to
their significant rear weight distribution
and large rear-brake assemblies.
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Diagonal Split
In some applications, the gain of the rear brakes may be low enough that a completely failed front
hydraulic circuit would not allow the vehicle to decelerate at an acceptable level. Therefore, an
alternate hydraulic design known as the diagonal split, or X-split, connects one front and one
diagonally opposed rear brake assembly (the left front brake and right rear brake, for example) to
each hydraulic circuit. In this fashion, regardless of which hydraulic circuit fails the remaining
braking capacity is the same.
Although this symmetry in design may appear highly desirable at first, it creates its own unique
performance compromise. Because the front brake gain and the rear brake gain are quite different
from one another, a failed diagonal circuit will create a brake force distribution that causes the
vehicle to pivot around the functioning front brake, resulting in a pull. This tendency can be
amplified by vehicle suspension parameters, but fundamentally all vehicles equipped with
diagonally split hydraulic circuits will try to change lanes under failed-circuit braking conditions.
In addition, the diagonal split implementation requires that two hydraulic lines be routed from the
master cylinder to the rear of the vehicle. For this reason, the more complex diagonal split is also
less preferred from a manufacturing and servicing perspective, but if the rear brakes are relatively
small, there may not be another option.
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If a vehicle has a significant percentage of
its weight carried on its front axle, the rear
brake gain may be too low to achieve an
appropriate level of deceleration during
failed-circuit braking. For this reason,
smaller vehicles with front-wheel drive
typically employ a diagonally split
hydraulic arrangement where the right
front is paired with the left rear, and the
left front is paired with the right rear.
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Brake Lines
A typically hydraulic circuit consists of both brake lines and brake hoses. The difference between
them is that brake lines are fabricated from mild steel, while brake hoses are made from flexible,
polymeric materials.
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Smaller vehicles with smaller
compliances require 3/16-inch brake
lines as shown on the left, while large
vehicles with excessive compliance
require 3/8-inch diameter sewer pipes
as shown on the right. If the brake line
selected is too small in diameter, it will
increase the brake system response
time. (Randall Shafer)
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Brake lines are typically attached to the master cylinder at one end, snaked around the vehicle like
spaghetti, and terminate at the brake hose somewhere near the moving parts of a vehicle’s
chassis. In order to protect them from damage, brake lines are typically made from mild steel and
are permanently bent to route their way around and along the vehicle’s underside. For additional
protection, they are usually held in place along the body with plastic clips or retainers—a brake line
hanging out in midair is just asking to get caught, snagged, or otherwise damaged by a passing
piece of road debris.
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Because a brake line failure can be
quite catastrophic, it makes sense to
route them up and out of the way of
any potential hazards. Clips and
brackets should be used to attach
individual lines to the vehicle body
structure adding yet another layer of
protection. (Randall Shafer/StopTech)
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Another benefit of mild steel construction is that it decreases the brake line’s compliance.
Remember back from Chapter 3 that when a hydraulic system is pressurized it wants to expand in
volume like a balloon. Mild steel helps add mechanical strength to the line, reducing its expansion
when subjected to brake fluid pressure.
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Flare Fittings
In order to form a leak-proof seal with the master cylinder and brake hose, a flare fitting is used. A
flare is essentially an upturned section of brake line material, which fits tightly against a seat
machined into a mating component, such as a master cylinder. When the integral tube nut is then
tightened, the force mechanically crushes the flare against the seat, forming a robust hydraulic seal.
While flare fittings are an elegant way of joining the hydraulic components together, there is a
limiting factor in their design and use. Because they require mechanical deformation to seal the
joint, the interface loses strength after repeated loosening and tightening cycles. In simpler terms, if
you take it apart it might not seal when you put it back together.
For this reason, flare fittings have a limited useful life before they must be cut off and reformed.
How many times is a function of vehicle-specific factors, but in general flares can begin to lose their
sealing capability after a few installation cycles. The helpful advice here is that you should not
disassemble a flared joint any more often than you must!
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Tube nuts are used to clamp the two
mating halves of the flare fitting
together. Because this is a
crush-to-seal interface, these nuts
should not be over tightened. Using
excessive force on the wrench may
seal the fitting at assembly, but
probably makes the flare unusable if
future servicing is required. (Randall
Shafer)
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Tips for Brake Line Installation
When installing new brake lines, you should make every attempt to match the stock vehicle routing.
However, if you’re forced to create a new routing, there are several factors that should be
considered before you start bending pipe.
1. Always use brake line the same diameter as stock. Most auto supply stores have several sizes to
choose from, so measure before you go or bring a sample of the old line with you.
2. Only use mild steel tubing for your brake line. You should never use copper or aluminum lines as
they are not strong enough for the high pressures found in a typical brake system!
3. Attempt to minimize the amount of brake line hanging out in midair. Wherever possible, keep the
line tight against the frame or body structure to reduce the opportunity for damage. Attaching the
line to the body using plastic clips or retainers is always a sound idea.
4. Route the brake line as far as possible from sources of extreme temperature, such as exhaust
manifolds and catalytic converters. The heat from these devices can boil the brake fluid in the line if
you get too close. If close quarters are unavoidable, be sure to install a thermal barrier or place an
insulating sleeve over the lines before final assembly.
5. If you’re routing brake lines in areas exposed to flying debris (along the vehicle underbody, for
example), you should consider installing spiral-wrapped brake line armor. This is cheap insurance!
6. Terminate the ends of the brake line with the appropriate flare fitting for your vehicle. Many auto
supply stores sell unbent lines with fittings already installed if you wish to pay a little bit extra for this
convenience.
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In this application, the brake lines are
forced to run close to the vehicle’s
exhaust manifold. To prevent fluid
boiling in the lines, they’re wrapped
with a heat-reflective shield that looks
much like tin foil. (Randall Shafer)
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Previous | Next
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This has been a sample page from
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High-Performance Brake Systems
Design, Selection, and Installation
by James Walker, Jr.
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High-Performance Brake Systems: Design, Selection, and
Installation gives you the knowledge to upgrade your brakes the
right way the first time. Author James Walker, Jr. doesn’t just tell
you what to do—he uses over 330 photos and plain English to
help you understand how and why your brake system works, what
each of the components does, and how to intelligently upgrade
your brakes for better performance. There are chapters showing
you how to choose and install the most effective rotors, calipers,
pads, and tires for your sports car, muscle car, race car, and
street rod. You will even find special sidebars detailing how each
upgrade will affect your ABS.
Brakes might be one of the most important, yet least understood,
vehicle systems. Brakes are relied upon day in and day out
without giving a second thought to their condition, let alone their
purpose, function, or design. Brake systems can be intimidating,
and they aren’t usually the first thing the average horsepower
junkie chooses to upgrade. But there’s no reason to wait until you
have a problem to learn how your brakes work. Whether you are
a casual enthusiast, a weekend warrior, or a professional racer,
this book will tell you everything you need to know about brakes.
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Click below to view a sample
page from each chapter
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Chap. 1 - Energy Conversion
Chap. 2 - Tires Stop the Car
Chap. 3 - System Design
Chap. 4 - Brake Balance
Chap. 5 - Pedal & Master Cyl
Chap. 6 - Brake Fluid
Chap. 7 - Lines and Hoses
Chap. 8 - Brake Calipers
Chap. 9 - Brake Pads
Chap. 10 - Brake Rotors
Chap. 11 - Sports Car Brakes
Chap. 12 - Race Car Brakes
Chap. 13 - Muscle Car Brakes
Chap. 14 - Street Rod Brakes
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8-1/2 x 11"
Softbound
144 pages
330+ color photos
Item: SA126
Price: $21.95
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Click here to buy now!
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This is a great book that any performance enthusiast will love!
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Other items you might be interested in
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Sport Compact Turbos & Blowers
While most owners of sport compacts can afford the simple bolt-ons
available, some owners want to take their modifications a step further.
There is intense competition to be the fastest, and quite often the only
way to win is to go to the next level – by installing a supercharger /
blower or turbocharger on your engine. This book is an enthusiast’s
guide to understanding and using turbochargers and superchargers
on sport compact cars.
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Price:
$18.95
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Buy more and save on shipping!
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MRE
PO Box 47
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