Bleeding/Flushing your BMW's Brakes, and Fabricating a Brake Bleeder Pressure Kit
by Steven Jay Bernstein, BMWCCA #83901
bleeder kit courtesy of Albert C. Cote
and Jon Lindsay
BMW recommends that the brake fluid system in their cars be flushed and bled
every year. Considering the importance of properly working brakes, this is the
single most important maintenance item and often the most overlooked.
The primary threats to a brake
system are water, dirt, and air. Brake fluid will absorb water and rust out the system
from within. It will also accumulate dirt, which can ruin the machined
surfaces in the brake master cylinder and the brake calipers. And air can sit in the
lines and cause the brakes to be spongy and less effective.
How often should you flush and bleed your brakes? Well, too much flushing your brakes
is like too much sex - there's no such thing. But seriously, for most applications, once a
year is adequate. But it is also dependent upon the amount of driving you do per
year, the amount of city versus highway driving you do, what kind of transmission your
car has, and how you use your brakes. The amount of money
spent on brake fluid is negligible (around $10-15, at most), so this shouldn't be a consideration.
For BMW owners attending performance driving schools, it is essential that this
service be performed competently and more frequently. Therefore, many of us do-it-yourself
mechanics prefer to do this service ourselves. While traditionally a two-person
job (one at the caliper, one in the driver's seat pumping the pedal), it is helpful to be
able to bleed brakes without the assistance of another. While I
have spent time on the weekend prior to a school servicing my brakes, I
have witnessed some driving school attendees bleed their brakes at a school between
track sessions in remarkably short order.
A pressure bleeder can be an essential tool in this job. Due to the way a
pressure tool works, it is imperative that a sufficient vacuum is achieved.
There are a number of commercial bleeders available on the aftermarket today.
These use the principle of sucking the fluid out of the caliper.
Unfortunately, general consensus is that they don't work very well. And amongst
my friends, we've tried all the ones you've read about. Therefore, Al Cote and
Jon Lindsay came up with a homemade tool which pressurizes the system
and pushes the fluid out of the caliper; it works quite well.
This FAQ is broken up into two parts. In the first part, I give some general guidelines
on how to bleed your brakes. In the second section, you'll find the bleeder parts list
and some associated instructions. Please remember that you
should consult your repair manual for any unique features about your BMW
that should be considered while performing this service. Examples of these unique
features are consideration of the ABS pump, and whether the car has a pressure
accumulator (or bomb).
- Some folks like to replace the fluid in the reservoir prior to flushing the lines. Some
have been insensitive enough to use kitchen utensils (like turkey basters) for this task.
This has been known to cause family unrest, and the author of this FAQ suggests you
find an alternate solution for the sake of your family. Truly, unless the fluid is
excessively dirty (very dark), this really isn't necessary.
- Assemble the bleeder as discussed by Al and Jon below.
- Put about a half quart of your favorite fluid into Tank #1 and tighten the lid. Refer to
your owner's manual for DOT specification if you are unsure. Unless you are a driving
school veteran and are attending more than 10 track days per year, I don't think it is
necessary to waste your money on racing fluids.
- Replace cap on the reservoir with Cap #3 and make sure you have a good seal.
- Pressurize Tank #1 to 10-15 psi. Check for leaks.
- Start with the caliper closest to the brake fluid reservoir. You might step on the
brake pedal once or twice to get the fluids moving.
- With the brake line closest to the fluid reservoir, open your caliper nipple. With the
first line, you want to completely replace the fluid in the reservoir with fresh fluid (unless
you did this prior to beginning). Close the nipple when the fluid is very clean and
when you see no more air bubbles.
- Work your way towards the longest brake line's caliper and repeat this procedure.
- For completeness, I like to repeat the procedure again to make sure there is no more
air trapped in the system. If you have replaced brake lines or rebuilt calipers, and have
introduced air into the system, it is imperative that you repeat the procedure. Otherwise,
use your best judgement.
- One other note is that it has been suggested that one start with the line furthest
from the fluid reservoir, but until someone can give me conclusive evidence explaining
why this works better, I'll stick with this for now.
- After you are finished, release the pressure in the tank by pushing down on the valve
release... then remove the caps.
Brake Bleeder Parts List
courtesy of Jon and Al
- TANK -- 1/2 gallon heavy duty (Nalgene) plastic tank. A dealer of
outdoor equipment should easily have such a tank.
They have many different styles and shapes. Your criteria should be
a tight seal on the screw cap, very solid construction, and a
relatively flat surface area on the top where you can mount a metal
tire valve. We used a tank with the following dimensions:
8"(h)x6"(w)x3"(d). It is most important that it be sturdy and
that most of the volume be air rather than fluid so that the pressure
remains relatively constant during the bleed. Use a good DOT-approved
fluid and put about a half quart in the tank. This tank was the most expensive
part ~ $9.50
- TANK -- 1quart tank into which old fluid is collected. You have
probably used a form of this in the past. At the same same outfitter
store, you can get a lighter duty quart jug for this purpose. Put another tire
valve in the cap, drill the valve out with a 1/4" bit, and run a
5/16" hose from the caliper nipple over a short section of stiff 1/4"
tubing which goes through the valve and down into the tank. A very
small hole drilled into the cap next to the valve will allow air to
escape. No more catching fluid in a wine bottle, if this one falls
over it is no problem, because it is a single piece.
- CAP -- Nalgene cap to temporarily fit the top of the reservoir in
place of the existing one with the sending unit. Find a Nalgene cap
about 1 3/4" in diameter to fit your BMW perfectly (a tight fit
here is essential). This part may take some trial and error and the
cap from one reservoir may be different from another.
- HOSE -- 2 feet of 3/8" OD x 1/4 ID vinyl hose-- to go from Tank #1
to the cap #3.
- HOSE -- 2 feet of 5/16" OD hose to drain the fluid into Tank #2.
- HOSE -- 9" of stiff 1/4" OD tubing to fit inside Tank #1 from the
drilled valve down to a corner in the tank (take a look at the
pesticide tank in your garage if you can't visualize this).
- VALVE STEMS -- 4 metal type screw valve stems -- these should
be available at
most car parts or tire stores. Drill out three of them as described for
Tank #2 above. The fourth one is not drilled out because it is
mounted in the cap of #1 above and is used to pressurize the system.
courtesy of Jon and Al
- Drill out three of the four valves with 1/4" bit (be careful and use a
- Drill out all three caps to accept the valves as well as a spot on the
shoulder of Tank #1. Mount undrilled valve in cap of Tank #1. Insert
1/4" stiff tubing into bottom end of one remaining valve and mount
valve on shoulder of Tank #1. Mount a remaining drilled valve into the hole
in cap which mounts on reservoir. Put 3/8" OD hose on the two valve
stems just described.
- Collector tank construction is described above. You can use a small
electric pump but a hand pump will do. Don't pressurize above 10
to 15 psi.
- Larger hoses will improve flow and a stop cock valve allows you to
fine tune your setup but is not necessary. A local observer suggested
a strap for the reservoir to ensure your reservoir doesn't decide to
lift off, another unnecessary precaution in my experience.
©2004 - Steven J. Bernstein All rights reserved.
Steven Jay Bernstein
Monday, 23 February 2004