(This article was obtained from the Honda Performance
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.
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
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.
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.