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Bleeding brakes is not one of my personal favorite jobs. There seems to be a bit of black
magic involved with the bleeding process. Sometimes it will work perfectly, and then other
times it seems like you end up with a lot of air in your system. The best strategy to
follow when bleeding your brakes is to repeat the procedure several times in order to make
sure that you have removed all the trapped air from the system.
After talking to many owners, it would
seem that there are more methods for bleeding brakes on a BMW than there are cures for the
common cold. Fortunately, I have polled many people and tried several different
solutions, and I think that I have come up with the best compromise solution. This
article is adapted from some other articles that I have written on bleeding brakes on
Porsches. The concepts are similar, and I have adapted them here along with pictures
for bleeding the brakes on your BMW.
There are currently three
methods of bleeding the brake system:
- Pressure Bleeding. This is where
you have a reservoir of brake fluid, and place a positive air pressure force on the
opposite side of the fluid, forcing it into the brake system.
- Vacuum Bleeding. This is where you
fill the reservoir, and then apply a vacuum at the bleeder nipple to pull fluid through
- Family Member Bleeding. This is
where you recruit the one family member or friend who owes you a favor and have them stomp
on the pedal repeatedly until the entire system is bled. Note that this has nothing
to do with the time that little Jimmy fell on the concrete and had to be rushed to the
The method that I've come up with combines
the first and the third methods described above. Basically, I advocate bleeding the
system with the pressure bleeder, and then using a family member to stomp on the pedal to
free up the proportioning valve. If the family member really owes you big time, you
will be the one stomping on the pedal, and they can spill brake fluid all over
start bleeding the system. Start with the right rear caliper, the one that's
located furthest away from the master cylinder. You should remove the rear wheels of
the car to easily get to the rear caliper (Figure 3). The
whole process is a heck of a lot easier if the car is off of the ground, and the wheels
have been removed. The front wheels can be turned for access to the calipers.
Bleed the right rear caliper by attaching a hose to the bleed nipple, placing it in a jar,
and then opening the valve with a wrench (a 7mm wrench is typically needed). A
bleeder nipple is shown in
4, and can be opened by turning it counter
clockwise. Let the fluid out until there are no more bubbles (Figure 5). If you don't have a pressure bleeder system, you need to find someone to
press on the pedal repeatedly to force fluid through the system. Another solution is
to get a check valve and place it on the nipple while you stomp on the pedal. This
will work for getting fluid into the system but you will still need a second person for
the final step - to make sure you have bleed the system completely. If your rear
caliper has two bleed nipples (some have one, others have two), bleed the lower one first.
When no more air bubbles come out, then
move to the next caliper. Bleed them in this order:
- Right Rear Caliper
- Left Rear Caliper
- Right Front Caliper
- Left Front Caliper
Bleeding in this
order will minimize the amount of air that gets into the system.
Repeat until you can no longer see any air bubbles
coming out of any of the calipers. Make sure that you don't run out of brake fluid
in your reservoir, or you will have to start over again. It is wise to start out
with about a 1/2 gallon of brake fluid in the pressure bleeder, and another 1/2 gallon on
the shelf in reserve. Depending upon your car, and the mistakes you may make, it's
wise to have an ample supply.
During the bleeding process,
its very easy to forget to check your master cylinder reservoir. As you are removing
fluid from the calipers, it will be emptying the master cylinder reservoir. If the
reservoir goes empty, then you will most certainly add some air bubbles in to the system,
and you will have to start all over. Keep an eye on the fluid level and dont forget
to refill it. Make sure that you always put the cap back on the reservoir. If the cap is
off, then brake fluid may splash out and damage your paint when the brake pedal is
released. If you are using a pressure bleeder system, make sure that you often check the
level of brake fluid in the bleeder reservoir so that you dont accidentally run dry.
If you are installing a new
master cylinder, its probably a wise idea to perform what is called a dry-bleed on
the workbench. This is simply the process of getting the master cylinder full of brake
fluid and wet. Simply add some brake fluid to both chambers of the master
cylinder, and pump it a few times. This will save you a few moments when bleeding the
Now, make sure that all the
bleeder valves are closed tightly. Disconnect the pressure system from the
reservoir. Now, get your family member to press down repeatedly on the brake pedal
at least five times, and then hold it down. Then open the bleeder valve on the right
rear caliper. The system should lose pressure, and the pedal should sink to the
floor. When the fluid stops coming out of the bleeder valve, close the valve, and
then tell your family member to let their foot off of the pedal. Do not let them
take their foot off until you have completely closed the valve. Repeat this motion
for each bleeder valve on each caliper at least three times. Repeat this entire
procedure for all the valves in the same order as described previously.
I recommend that you use this
procedure as a final step, even if you are vacuum or pressure bleeding. The high force
associated with the pressure from the brake pedal can help free air and debris in the
lines. If the brake fluid doesnt exit the nipple quickly, then you might have a clog
in your lines. Brake fluid that simply oozes out of the lines slowly is a clear indication
that your rubber lines might be clogged and constricted. Dont ignore these warning
signs check out the brake lines while you are working in this area.
Then, let the car sit for about
10 minutes. Repeat the bleeding process at each corner. The pedal should now
feel pretty stiff.
If the pedal still feels
spongy, make sure that you have the proper adjustment on your rear calipers or drum
shoes. Also, you may need a new master cylinder, have a leaky caliper, or have old
spongy flexible brake lines.
Another important thing to
remember is that brake fluid kills paint jobs that is. Brake fluid spilled on paint
will permanently mar the surface, so be very careful not to touch the car if you have it
on your hands and clothing. This of course, is easier said then done. Just be aware of
this fact. Rubber gloves help to protect yourself from getting it on your hands and your
paint. If you do get a spot on your paint, make sure that you blot it with a paper towel -
dont wipe or smear it. Its also important not to try to clean it off with any
chemical or other cleaning solutions.
|| There are
few little tricks that you can use when changing your brake fluid. The company ATE makes a
brake fluid called SuperBlue that comes in two different colors (Figure 6). Its
a smart idea to fill your reservoir with a different colored fluid, and then bleed the
brakes. When the new colored fluid exits out of the caliper, you will know that you have
fresh fluid in your system. Make sure that you use DOT 3 or DOT 4 brake fluid in your car.
Some of the later model BMWs with anti-lock braking systems require the use of DOT 4. The
use of silicone DOT 5 fluid is not recommended for street use.
You should also routinely flush and
replace your brake fluid every two years. Deposits and debris can build up in the lines
over time and decrease the efficiency of your brakes. Regular bleeding of your system can
also help you spot brake problems that you wouldnt necessarily notice simply by
driving the car. Also, never reuse brake fluid - always use new fresh fluid.
In addition, don't use brake fluid that has come from an empty can that has been
sitting on the shelf. The brake fluid has a tendency to absorb moisture when sitting
on the shelf. This moisture 'boils' out of the brake fluid when you start using the
brakes, and results in a spongy pedal.
Well that's about all it
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