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How to
Bleed Brakes – The Right Way
by John Comeskey of
SPS
and James
Walker, Jr. of scR motorsports
The role of the brake fluid within
the braking system is to transfer the force from the master cylinder to the
corners of the car…and a vital characteristic of brake fluid that allows it to
perform its task properly is its ability to maintain a liquid state and resist
compression. In order to keep the fluid in top condition, many enthusiasts have
been taught to “bleed their brakes” but many have never stopped to ask the
question “why?
Why Bleed the
Brakes?
The term "bleeding the brakes"
refers to the process in which a small valve is opened at the caliper (or wheel
cylinder) to allow controlled amounts of brake fluid to escape the system. (When
you think about it, "bleeding" may appear to be a somewhat graphic term, but it
aptly describes the release a vital fluid.)
We bleed the brakes to release air
that sometimes becomes trapped within the lines. Technically, "air" only enters
the lines if there is a compromise of the system's sealing (as when flex lines
are removed or replaced), because when fluid boils, it will instead create
"fluid vapor." Vapor in the brake fluid, like air, will create an efficiency
loss in the braking system. However, for the sake of simplicity we use the term
"air" throughout this article to describe both air and fluid vapor.
When air (or vapor) becomes present
within the lines, it creates inefficiencies within the system because, unlike
liquid, air can be compressed. So when enough air fills the lines, input at the
pedal merely causes the air to compress instead of creating pressure at the
brake corners. In other words, when air is present within the system, the
efficiency and effectiveness of the braking system is reduced. Usually, a small
amount of air within the brake system will contribute to a "mushy" or "soft"
pedal (since less energy is required to compress the air than is required to
move fluid throughout the brake lines.) If enough air enters the brake system,
it can result in complete brake failure.
So how does air enter the lines in
the first place? Sometimes, it can be the result of a service procedure or an
upgrade – such as replacing the stock flex lines with stainless steel braided
lines. But often it is the result of high temperatures that cause brake fluid
components to boil, thus releasing gasses from the boiling fluid into the brake
hydraulic system.
Brake Fluid
Selection
This leads one to contemplate the
type of liquid that is used as brake fluid. In theory, even simple water would
work – since, being a liquid, water cannot be compressed. However, it is
important to remember that the fundamental function of the braking system is to
convert kinetic energy into heat energy through friction. And the reality of
this process is that certain parts of the braking system will be exposed to very
high temperatures. In fact, it is not uncommon to see rotor temperatures during
a race as high as 1200 degrees Fahrenheit – which can raise the temperature of
the brake fluid to well over 300 degrees Fahrenheit. Since the boiling point of
water is 212 degrees Fahrenheit, it is easy to see that water within the brake
system could boil easily – and therefore release gases into the brake pipes –
which would reduce the efficiency of the system. (Water would also present a big
problem in cold weather if it froze to ice!)
The "obvious" solution to this
problem is to utilize a fluid that is less sensitive to temperature extremes.
Hence the development of "brake fluid." However, there unfortunately is no such
thing as a "perfect" brake fluid. And like most things in the world, the
addition of certain beneficial characteristics usually brings tradeoffs in other
areas. In the case of brake fluid, we generally must balance the fluid's
sensitivity to temperature against its cost and its impact upon other components
within the system.
Stated more bluntly, it is possible
to reduce a fluid's sensitivity to temperature by varying the ingredients of the
fluid. However, certain combinations of ingredients can significantly increase
the cost of the fluid and may react with OEM materials to damage seals and
induce corrosion throughout the braking system.
The chemical composition and
minimum performance requirements of the fluid are generally indicated through a
rating such as "DOT3," DOT4," or "DOT5." The DOT-rating itself is assigned after
a series of government tests. However, this rating is NOT intended to indicate
boiling points, even though higher DOT ratings generally do correspond with
higher boiling points. Perhaps more importantly, the DOT rating does indicate
the base compound of the brake fluid - which allows manufacturers to specify
fluid types which are less likely to react negatively to known materials used
within a particular braking system.
The greatest irony about brake
fluid, however, is the fact that the chemical compositions that tend to be less
sensitive to temperature extremes also tend to attract and absorb water! So even
though the fluid itself is unlikely to boil (most glycol-based DOT3 fluids have
a "dry boiling point" around 400 degrees Fahrenheit,) the water that it absorbs
over time tends to boil easily (at 212 degrees Fahrenheit.) It is this
characteristic of absorbing moisture that leads to the measure known as the "wet
boiling point." The wet boiling point is the equilibrium boiling point of the
fluid after it has absorbed moisture under specified conditions. Because brake
fluid will absorb moisture through the brake system's hoses and reservoir,
evaluation of the wet boiling point is employed to test the performance of used
brake fluid and the degradation in it's performance. (And it is why we still
need to bleed the brakes frequently on racecars, even though we use racing fluid
that costs upwards of $75 per bottle!) The lesson: do NOT expect to avoid
bleeding your brakes just because you bought expensive brake fluid.
As one might guess, "racing" fluids
will use relatively "aggressive" chemical compositions which will tend to have
higher wet boiling points and higher costs, while the average street fluids will
use more conservative compositions which will have lower wet boiling points and
lower costs. In some cases – such as a purpose-built racecar – the tradeoffs of
using the expensive racing fluid is outweighed by the competitive advantages.
But for the average driver – whose driving style is less likely to induce brake
temps as high as those seen on the track – the costs of the fluids and potential
wear-and-tear factors upon system components may justify the use of a more
conservative fluid with a lower wet boiling point.
How-To
So, now that you understand the
need behind bleeding your brakes, let us present just one procedure that can be
utilized when servicing your own car. Note that unless you are replacing your
master cylinder, the procedure is the same whether you have a vehicle equipped
with ABS or not…
Supplies Required
You will need the following tools:
· Box-end wrench suitable for your car's bleeder screws. An offset head design
usually works best.
· Extra brake fluid (about 1 pint if you are just bleeding, about 3 if you are
completely replacing).
· 12-inch long section of clear plastic tubing, ID sized to fit snugly over your
car's bleeder screws.
· Disposable bottle for waste fluid.
· One can of brake cleaner.
· One assistant (to pump the brake pedal).
Vehicle
Preparation and Support
1. Loosen the lug nuts of the road
wheels and place the entire vehicle on jackstands. Be sure that the car is
firmly supported before going ANY further with this procedure!
2. Remove all road wheels.
3. Install one lug nut backward at each corner and tighten the nut against the
rotor surface. Note that this step is to limit caliper flex that may distort
pedal feel.
4. Open the hood and check the level of the brake fluid reservoir. Add fluid as
necessary to ensure that the level is at the MAX marking of the reservoir. Do
not let the reservoir become empty at any time during the bleeding process!
Bleeding Process
1. Begin at the corner furthest
from the driver and proceed in order toward the driver. (Right rear, left rear,
right front, left front.) While the actual sequence is not critical to the bleed
performance it is easy to remember the sequence as the farthest to the closest.
This will also allow the system to be bled in such a way as to minimize the
amount of potential cross-contamination between the new and old fluid.
2. Locate the bleeder screw at the rear of the caliper body (or drum brake wheel
cylinder.) Remove the rubber cap from the bleeder screw – and don't lose it!
3. Place the box-end wrench over the bleeder screw. An offset wrench works best
– since it allows the most room for movement.
4. Place one end of the clear plastic hose over the nipple of the bleeder screw.
5. Place the other end of the hose into the disposable bottle.
6. Place the bottle for waste fluid on top of the caliper body or drum assembly.
Hold the bottle with one hand and grasp the wrench with the other hand.
7. Instruct the assistant to "apply." The assistant should pump the brake pedal
three times, hold the pedal down firmly, and respond with "applied." Instruct
the assistant not to release the brakes until told to do so.
8. Loosen the bleeder screw with a brief ¼ turn to release fluid into the waste
line. The screw only needs to be open for one second or less. (The brake pedal
will "fall" to the floor as the bleeder screw is opened. Instruct the assistant
in advance not to release the brakes until instructed to do so.)
9. Close the bleeder screw by tightening it gently. Note that one does not need
to pull on the wrench with ridiculous force. Usually just a quick tug will do.
10. Instruct the assistant to "release" the brakes. Note: do NOT release the
brake pedal while the bleeder screw is open, as this will suck air back into the
system!
11. The assistant should respond with "released."
12. Inspect the fluid within the waste line for air bubbles.
13. Continue the bleeding process (steps 11 through 16) until air bubbles are no
longer present. Be sure to check the brake fluid level in the reservoir after
bleeding each wheel! Add fluid as necessary to keep the level at the MAX
marking. (Typically, one repeats this process 5-10 times per wheel when doing a
‘standard' bleed.)
14. Move systematically toward the driver – right rear, left rear, right front,
left front - repeating the bleeding process at each corner. Be sure to keep a
watchful eye on the brake fluid reservior! Keep it full!
15. When all four corners have been bled, spray the bleeder screw (and any other
parts that were moistened with spilled or dripped brake fluid) with brake
cleaner and wipe dry with a clean rag. (Leaving the area clean and dry will make
it easier to spot leaks through visual inspection later!) Try to avoid spraying
the brake cleaner DIRECTLY on any parts made of rubber or plastic, as the
cleaner can make these parts brittle after repeated exposure.
16. Test the brake pedal for a firm feel. (Bleeding the brakes will not
necessarily cure a "soft" or "mushy" pedal – since pad taper and compliance
elsewhere within the system can contribute to a soft pedal. But the pedal should
not be any worse than it was prior to the bleeding procedure!)
17. Be sure to inspect the bleeder screws and other fittings for signs of
leakage. Correct as necessary.
18. Properly dispose of the used waste fluid as you would dispose of used motor
oil. Important: used brake fluid should NEVER be poured back into the master
cylinder reservoir!
Vehicle Wrap-Up
and Road Test
1. Re-install all four road wheels.
2. Raise the entire vehicle and remove jackstands. Torque the lug nuts to the
manufacturer's recommended limit. Re-install any hubcaps or wheel covers.
3. With the vehicle on level ground and with the car NOT running, apply and
release the brake pedal several times until all clearances are taken up in the
system. During this time, the brake pedal feel may improve slightly, but the
brake pedal should be at least as firm as it was prior to the bleeding process.
4. Road test the vehicle to confirm proper function of the brakes. USE CAUTION
THE FIRST TIME YOU DRIVE YOUR CAR AFTER MODIFICATION TO ENSURE THE PROPER
FUNCTION OF ALL VEHICLE SYSTEMS!
How Often do I
Need to Bleed My Brakes?
In closing, here are a few rules of
thumb to help you to determine the proper bleeding interval for your particular
application:
1. Under normal operating
conditions, and without brake system modifications, typical OEM braking systems
have been designed to NOT require bleeding for the life of the vehicle unless
the system is opened for repair or replacement. If you're just driving around
town or on the highway to work, there is really no need to bleed! There are a
few European vehicles which do recommend replacement on a semi-regular basis for
other reasons though, so be sure to check in your owner's manual or at your
service center for your particular application.
2. Those who choose to autocross or drive in a sporting manner may choose to
upgrade their brake fluid and bleed on an annual basis – this is a good ‘start
of the season' maintenance item for low-speed competitors.
3. If your car sees significant amounts of high-speed braking, or if you choose
to participate in driver schools and/or lapping sessions, bleeding prior to each
event is a sound decision. More intense drivers at these events may choose to
skip right past this step and on to #4…
4. Finally, dedicated race cars should be bled after every track session.
