Here's an interesting post from a BMW owner's list with some additional food
for thought:
Brake Fluid Facts
by Steve Wall
As a former materials engineering supervisor at a major automotive brake
system supplier, I feel both qualified and obligated to inject some material
science facts into the murky debate about DOT 5 verses DOT 3-4 brake fluids.
The important technical issues governing the use of a particular
specification brake fluid are as follows:
Fluid compatibility with the brake system rubber, plastic and metal
components.
Water absorption and corrosion.
Fluid boiling point and other physical characteristics.
Brake system contamination and sludging.
Additionally, some technical comments will be made about the new brake fluid
formulations appearing on the scene.
First of all, it's important to understand the chemical nature of brake
fluid. DOT 3 brake fluids are mixtures of glycols and glycol ethers. DOT 4
contains borate esters in addition to what is contained in DOT 3. These
brake fluids are somewhat similar to automotive anti-freeze (ethylene
glycol) and are not, as Dr. Curve implies, a petroleum fluid. DOT 5 is
silicone chemistry.
Fluid Compatibility
Brake system materials must be compatible with the system fluid.
Compatibility is determined by chemistry, and no amount of advertising,
wishful thinking or rationalizing can change the science of chemical
compatibility. Both DOT 3-4 and DOT 5 fluids are compatible with most brake
system materials except in the case some silicone rubber external components
such as caliper piston boots, which are attacked by silicon fluids and
greases.
Water absorption and corrosion
The big bugaboo with DOT 3-4 fluids always cited by silicone fluid advocates
is water absorption. DOT 3-4 glycol based fluids, just like ethylene glycol
antifreezes, are readily miscible with water. Long term brake system water
content tends to reach a maximum of about 3%, which is readily handled by
the corrosion inhibitors in the brake fluid formulation. Since the
inhibitors are gradually depleted as they do their job, glycol brake fluid,
just like anti-freeze, needs to be changed periodically. Follow BMW's
recommendations. DOT 5 fluids, not being water miscible, must rely on the
silicone (with some corrosion inhibitors) as a barrier film to control
corrosion. Water is not absorbed by silicone as in the case of DOT 3-4
fluids, and will remain as a separate globule sinking to the lowest point in
the brake system, since it is more dense.
Fluid boiling point
DOT 4 glycol based fluid has a higher boiling point (446F) than DOT 3
(401F), and both fluids will exhibit a reduced boiling point as water
content increases. DOT 5 in its pure state offers a higher boiling point
(500F) however if water got into the system, and a big globule found its way
into a caliper, the water would start to boil at 212F causing a vapor lock
condition [possible brake failure -ed.]. By contrast, DOT 3 fluid with 3%
water content would still exhibit a boiling point of 300F. Silicone fluids
also exhibit a 3 times greater propensity to dissolve air and other gasses
which can lead to a "spongy pedal" and reduced braking at high altitudes.
DOT 3 and DOT 4 fluids are mutually compatible, the major disadvantage of
such a mix being a lowered boiling point. In an emergency, it'll do.
Silicone fluid will not mix, but will float on top. From a lubricity
standpoint, neither fluids are outstanding, though silicones will exhibit a
more stable viscosity index in extreme temperatures, which is why the US
Army likes silicone fluids. Since few of us ride at temperatures very much
below freezing, let alone at 40 below zero, silicone's low temperature
advantage won't be apparent. Neither fluids will reduce stopping distances.
With the advent of ABS systems, the limitations of existing brake fluids
have been recognized and the brake fluid manufacturers have been working on
formulations with enhanced properties. However, the chosen direction has not
been silicone. The only major user of silicone is the US Army. It has
recently asked the SAE about a procedure for converting from silicon back to
DOT 3-4. If they ever decide to switch, silicone brake fluid will go the way
of leaded gas.
Brake system contamination
The single most common brake system failure caused by a contaminant is
swelling of the rubber components (piston seals etc.) due to the
introduction of petroleum based products (motor oil, power steering fluid,
mineral oil etc.) A small amount is enough to do major damage. Flushing with
mineral spirits is enough to cause a complete system failure in a short
time. I suspect this is what has happened when some BMW owners changed to
DOT 5 (and then assumed that silicone caused the problem). Flushing with
alcohol also causes problems. BMW brake systems should be flushed only with
DOT 3 or 4.
If silicone is introduced into an older brake system, the silicone will
latch unto the sludge generated by gradual component deterioration and
create a gelatin like goop which will attract more crud and eventually plug
up metering orifices or cause pistons to stick. If you have already changed
to DOT 5, don't compound your initial mistake and change back. Silicone is
very tenacious stuff and you will never get it all out of your system. Just
change the fluid regularly. For those who race using silicone fluid, I
recommend that you crack the bleed screws before each racing session to
insure that there is no water in the calipers.
New developments
Since DOT 4 fluids were developed, it was recognized that borate ester based
fluids offered the potential for boiling points beyond the 446F requirement,
thus came the Super DOT 4 fluids - some covered by the DOT 5.1 designation -
which exhibit a minimum dry boiling point of 500F (same as silicone, but
different chemistry).
Additionally, a new fluid type based on silicon ester chemistry (not the
same as silicon) has been developed that exhibits a minimum dry boiling
point of 590F. It is miscible with DOT 3-4 fluids but has yet to see
commercial usage.
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