LONG/SPONGY PEDAL - the compressibility of the silicone-based fluid is up to
three times that of the more common glycol-based types, so needs more pedal
travel to actuate. Its viscosity is twice that of the glycol-based fluid -
meaning it is thicker. This equates to slow fill rates that can trap air,
and results in bleeding difficulties.
SUDDEN LOSS OF BRAKES - Entrapped air suffers gasification at relatively low
temperatures, causing a vapour-lock effect. It's immiscibility causes any
free water caught in the system to boil at relatively low temperatures -
producing a vapour-lock. Glycol-based fluids absorb water (the water
dissolves into the fluid), and although this will reduce the boiling point,
it is unlikely to cause severe vapour-lock at low temperatures.
HANGING ON - In disc brake systems, the sole mechanism for returning the
pads to their normal 'not in use' position away from the disc is the
tendency of the seals to recover to their 'at rest' attitude once the pedal
is released. The low lubricity of the silicone-based fluid works against
this recovery; it's high viscosity amplifying this effect
> DOT 3/4/5.1 Verses DOT 5. Which brake fluid should I use?
1.. Fluid compatibility with the brake system rubber, plastic and
metal components.
2.. Water absorption and corrosion.
3.. Fluid boiling point and other physical characteristics.
4.. Brake system contamination and sludging.
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) ,. 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 5.1 glycol based fluid has a higher boiling point (520F) than DOT
4 (446F), DOT3(401F) and all 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 3/4/5.1 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 /4/5.1
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 520F (better than
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.
regards
Ivor Phillips
> What's not to like about silicone brake fluid? The stuff is compatible
with
> all seal materials, does not absorb atmospheric moisture, does not
dissolve
> paint, is rated for higher temperature operation, barely supports
> combustion, does not need to be replaced, and is otherwise virtually
inert.
> Except for price silicone brake fluid seems to be superior to all the
other
> choices in every respect.
>
>
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