By Mike Perkins
BMW CCA/Houston Chapter Technical Advisor
Owner - Bavarian Machine Specialties
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months, I have started to see more anti-lock brake system
related brake problems. For the past decade, this amazingly
effective system has, for the most part, been trouble free.
As a reminder to club members, I offer some basic brake suggestions
to avoid trouble.
and foremost is to regularly flush the brake fluid system.
BMW recommends that brake fluid be changed every two years.
Since the advent of ABS in 1985, BMW recommends that all models
use Dot 4 brake fluid (such as Castrol GTLMA). This brake
fluid, as is the case with all Dot 4 brake fluid brands, is
hygroscopic, meaning that it readily absorbs moisture when
exposed to air. Moisture lowers the boiling point of the fluid
and can cause brake fade. It has been estimated that in Houston,
two year old brake fluid will contain anywhere from 4 to 8%
water. This small amount will lower the boiling point by more
than 25 to 30% which could cause compromised stopping power
when it is needed most. In addition, moisture in the system
can cause corrosion in the bores of system components, eventually
leading to premature seal wear (i.e. leaking calipers, master
cylinders, etc). Most of the ABS related failures that I have
encountered can be traced to moisture contamination which
leads to corrosion that can make the ABS valves stick. This
usually requires expensive repairs that could have been avoided
by periodic flushing.
has more moisture in the atmosphere than do most places in
the country. Because of this, I recommend brake fluid be tested
for moisture content at least once a year. Flushing may be
wise to do on a yearly basis, especially if the car is exposed
to severe driving conditions (i.e. constant freeway stop and
go, or racing/ auto-crossing). In fact, if a car is to be
used at the track at all, I recommend the use of ATE Super
Blue, which is still a Dot 4 fluid but has a higher boiling
point than normal Dot 4 fluid.
we are on the subject of alternative brake fluids, I still
receive many questions regarding the use of silicone brake
fluid. Anti-lock brake systems which are of the integral type
that pumps fluid under high pressure (such as all BMW's) should
not use silicone brake fluid. Silicone fluid does not provide
the correct amount of lubrication for the essential working
parts of a master cylinder, calipers, or wheel cylinders.
In addition, silicone fluid has a tendency to aerate when
put in a pressurized situation. Any air bubbles in the brake
lines will cause the pedal to sink drastically. Silicone fluid
also repels moisture. Because many ABS controls and valves
are made of stainless steel, attraction toward other ABS components
made of steel happens at an accelerated rate. This could cause
sticking or failure of the ABS unit.
regular flushes are important, proper flushing is crucial.
On an ABS equipped car, this requires cycling the ABS unit
during flushing. The ABS pump holds fluid that will not be
flushed unless the pump is run. Another way to insure an effective
flush on all brake systems is to clean out the brake fluid
reservoir. This should be done during every flush by removing
the reservoir from the master cylinder for clean out with
brake cleaner. You will notice a film of residue that has
settled on the bottom of the reservoir; if this is not cleaned
out you will simply flush this contaminate into the system.
recommendation is something that is often overlooked by both
novice and expert alike. When working on brakes for any reason,
do not force the caliper pistons back into the caliper without
first releasing the caliper bleeder screw. Not following this
simple step is inviting certain disaster and can have a dramatically
negative effect on the longevity of the brake system. The
calipers are located at the lowest point of the hydraulic
system and tend to collect particles released from suspension
in the fluid. Compressing the pistons forces this contamination
back through the system causing other component failures.
Always open the bleeder screw allowing contaminated fluid
to escape before pushing the piston inward. Remember, it is
better to strip a bleeder screw than to contaminate the ABS