How
to Bleed Brakes – The Right Way
by John
Comeskey of SPS http://www.spswebpage.com
and James
Walker, Jr. of scR motorsports http://www.teamscR.com
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.