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I have finished a FAQ on silicone brake fluid. It is not too large, so I am
just including it below. If you find it to be of interest you may wnat to
keep it. I have no doubt that I have left things out. If you have any
questions that I have not answered, please let me know and I will try to
update my text.
My apologies if this is so long that you find it annoying to receive.
Jim
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Silicone.faq
Background:
Modern standard brake fluids fall into two classifications: DoT-3 and DoT-4.
The difference is in the boiling point. DoT-4 is higher boiling point than
DoT-3. The boiling point must be no lower than the specified temperature
when the fluid has up to a certain (small) amount of moisture in it. The
boiling point comes down as the water content goes up.
The motivation for the development of Silicone Brake Fluid came from the
military in the late 1960s. At that time the military used 3 kinds of brake
fluid: one for tropical climates, one for temperate and arctic climates, and
one for storage--to prevent corrosion. The military was interested in
eliminating the expense and hassle of having to change fluids any time
vehicles were taken in and out of storage or moved around the world.
A Silicone Brake Fluid became available in the early 70s under a number of
labels, but the only manufacturers were Dow-Corning and General Electric. A
formulation was finally achieved in the late 70s that satisfied DoT criteria
and this became DoT-5 Silicone Brake Fluid.
I've been using silicone DoT-5 for over 16 years now in a multitude of
cars--mostly air-cooled VW's. Since changing over to exclusive use of DoT-5
in 1979 I have almost completely eliminated the need to rebuild disk brake
calipers and master cylinders. I have this in cars that are daily drivers
and in cars that get driven less than once a year. I figure it pays for
itself in every car every year, since I had found myself doing some kind of
brake maintenance almost every year on my older cars.
Altogether, I have installed DoT-5 Silicone Brake Fluid in more than 50
cars. Of all those I have had two problems. One was a Volvo where the
master cylinder failed a few months later, and inspection revealed that a
piece of dirt had come loose and lodged against one of the cups wearing a
grove in it. The second was a TR-3 that had been rebuilt by the owner and I
could never get it bled properly; that car finally worked when it was taken
to a shop that understood British cars--they replaced all the brake parts
and put DoT-5 back in with no problems.
The benefits:
1) Silicone brake fluid is non-hydroscopic. That is, it does not absorb
water out of the air. Normal (DoT-3/4) fluids are all hydroscopic and their
properties--boiling point, viscosity, viscosity as a function of
temperature, and propensity to cause rusting of the metal parts of the brake
system--change as the water content changes, all in the undesirable
direction. It is a little known fact that ALL DoT-3/4 fluids need to be
replaced periodically; every 2 years is probably adequate, every 4 years is
definitely too long. Once properly installed, DoT-5 Silicone Brake Fluid
never needs to be replaced.
2) Silicone Brake Fluid is an excellent lubricant for metal-to-rubber and
metal-to- plastic contact points. This is in stark contrast to the Girling
LMA DoT-4 which I tried as an affordable alternative to the hard to get and
expensive OEM ATE Blue DoT-4. I found that master cylinder primary and
secondary cups were wearing out in typically 2 years with Girling LMA. This
was consistent with the "squeaky clean" pedal feel that I found with this
fluid. While Silicone is poor for metal-to metal contact, there is very
little of that inside a typical hydraulic brake system.
3) Silicone DoT-5 does not thicken in the cold temperatures that we get
occasionally in this part of the country. At -25 F even dry DoT-3/4 gets
quite thick and brakes get sluggish; wet DoT-3/4 is even worse. I realize
this may not be a problem for most of you.
4) Silicone Brake Fluid does not attack paint like DoT-3/4 fluids do. So
fluid that leaks out on your car will not destroy your paint and promote
rusting.
The drawbacks:
1) Silicone Brake Fluid is expensive. It costs maybe 4 times as much as
regular fluids, plus you need to use a lot of it to do the job right. I
figure about 1.5 quarts for one of my VWs. If you have taken everything
apart and don't need to do a lot of flushing, the job can be done with a bit
more than half a quart.
2) Time consuming to install. You can't just pour it in and bleed the
brakes. I figure two people for two hours, one hour if you know exactly
what you are doing and have everything ready.
3) Not particularly environmentally friendly. It should not be flushed
down the drain or allowed to get into the water table. While I don't think
it has any particular noxious properties, it is persistent in the
environment (does not break down like DoT-3/4--ever smelled an old brake
part from the junk yard?) so take care how you dispose of it. Dow- Corning
recommends disposal by incineration. I have avoided the problem by just
recycling the fluid I use for flushing. More on this later.
4) Silicone fluid might not be suitable for new cars with ABS. This is not
at all clear at this time. The manufacturers are adamant about just using
their fluid. This may be due to the fact that ABS uses a pump which
requires the metal-to-metal lubricity of DoT-3/4, or it might just be a
marketing ploy. At this time I recommend caution.
Note that it IS safe to ignore the very specific warning in the Robert
Bentley manuals not to use Silicone Brake Fluid in the VW water cooled cars.
These cars have brakes that have the same materials made by the same
manufacturers as the air cooled cars. I have done a dozen of these cars;
Dow-Corning has done hundreds. Most of these brake components were made by
ATE, Afred Teves, GMBH, Germany. Teves makes brakes components and brake
fluid. Last I heard they did not market a Silicone Brake Fluid, but they DO
buy DoT-5 Silicone Brake Fluid from Dow-Corning to use as an assembly fluid
so their parts won't rust on the shelf. Why this warning exists remains a
mystery.
5) Silicone Brake Fluid that you spill on your car will make it hard to
repaint that area unless it is cleaned off very carefully. It does not
clean up with water.
Installing Silicone Brake Fluid in your car:
There are several possible ways to do this. They are all aimed at filling
your system with 100% Silicone fluid. The quality of the job you do will be
proportional to the degree that you rid your car of the old fluid. If you
intend to do this job, please read through the whole FAQ and make sure you
understand EVERYTHING before you start. Note that these instructions assume
that you are already familiar with standard and appropriate brake system
practice. If you are not, you will need to find someone who is to help you
or study up on these things first.
1) If you are a manufacturer, all you need to do is clean out your existing
pressure bleeder equipment and fill it with Silicone Brake fluid. Then just
install it in your cars the same as you did before. This is obviously the
easiest and cheapest way to do it. Unfortunately, it might add $5 to the
manufacturer's cost, so we are not likely to ever see it happen. It is in
general use in some fleet application, the military, the post office (I
think?), and Harley-Davidson Motorcycles.
2) If you have an old car that previously had DoT-3/4 in it, you should get
the hydraulic system into good shape first, then flush DoT-5 through it to
replace as much of the old DoT-3/4 fluid as possible. Since the Silicone
does not mix with the old fluid, the point of flushing is to create as much
turbulence as possible in the volumes where the old fluid is present in
order to create a "slurry" and carry out the old fluid, as well as any
accumulated dirt, water, and rust. This means that you must pump the brake
pedal HARD on the down stroke, and let it up relatively easily; do not slam
the pedal into the end of its travel--that could damage the master cylinder.
It is not necessary to close the bleed valve on th upstroke, but you should
be bleeding through a clear plastic tube that leads to the bottom of a
container that will catch your flushing fluid and prevent air from being
sucked back up into the hydraulic brake system.
When you flush the system one wheel at a time, you will observe that the
fluid first comes out muddy (rust) or amber (typical DoT-3/4 color.) Then
the color will turn cloudy. This is the slurry, or mixture, of the Silicone
Brake Fluid and the old fluid--sort of like oil and vinegar. Eventually the
cloudyness will go away and be replaced by the color of the DoT-5. Once
this happens you have successfully flushed that wheel and can move on to the
next.
Turbulent flushing is easy to achieve in the master cylinder and in drum
brake cylinders. However, it can be quite difficult in some designs of disk
brake calipers. Since one of the principle benefits of Silicone Brake Fluid
is the prevention of frozen calipers, they deserve some special care.
Here's the problem--Silicone Brake Fluid is lighter than regular fluid, it
floats on top. Thus the large cavities in a brake caliper may still be half
full of old fluid while the flush fluid appears to be coming out clean.
The best way to attack this problem is to remove the calipers and rebuild
them. The best way to get rid of the old fluid is to flush the disassembled
caliper parts with water because standard fluid is completely soluble in
water. If the caliper is particularly dirty or greasy I will scrub it with
soap and water, then rinse it with clean water, blow out all passages and
finally rinse in alcohol and blow everything out. Dry it quickly to avoid
rusting, and make sure to remove ALL traces of water. Let dry overnight in a
warm, dry place. As a rebuilding lubricant use only Silicone Brake Fluid,
or another silicone lubricant, not DoT-3/4 or any petroleum containing
product. Then make some brake line caps from old brake hose female ends;
just cut off the rubber hose and weld the metal closed. Use these caps to
allow the rest of the brake system to be flushed and bled, removing one cap
at a time to flush its line until everything is clean.
Now you can re-install your clean, rebuilt caliper on the car and let it
slowly fill with DoT-5 until air stops coming out the bleeder valve. Keep
in mind that it would be silly to go to all the trouble of rebuilding a
caliper and then go mount it on the car and flush the old dirty fluid out
through it.
If your calipers are in good shape, you can, as an alternative, unbolt the
calipers from the steering knuckles and hang them upside down so the bleeder
valves are on the bottom. Place a slab of wood or something between the
brake pads to prevent the pistons from popping out of their bores when your
helper pumps the pedal. Now the standard fluid will flush out selectively
(remember, the standard fluid is heavier.) Aircooled VWs, made 8/71 or
later, came with a second bleeder valve on the bottom of the caliper. This
makes these calipers easy to flush on the car. Just do all the flushing via
the bottom bleeder valve, then finish off by bleeding any air off the top.
These late calipers also have the advantage that it is easy to come back a
week, a month, or a year later, and bleed just a bit of fluid out to
eliminate the last drops of old fluid which is bound to persist.
Recycling old fluid:
The Silicone is expensive, but, unlike DoT-3/4 it does not absorb water.
You can let your DoT-5 flushing residue stand until the silicone and
standard fluid separate (the silicone is lighter and rises to the top.)
Decant the silicone portion, then put it in a clean container and mix it
vigorously with an equal amount of clean water. Any DoT-3/4 fluid will be
absorbed into the water and, after separation, the recycled Silicone Brake
Fluid can again be poured off the top.
I decant the Silicone Brake Fluid from the DoT-3/4 once, and twice from
clean water, filtering each time. As a final step, I heat the decanted
silicone fluid to about 110 C to verify that there is no water in the end
product.
If you do this and keep it clean (NO PETROLEUM OIL!) the recycled silicone
fluid can be reused to flush the next car. I typically use recycled
Silicone Brake Fluid to flush a car, then do a final bleed with virgin DoT-5
Silicone Brake Fluid. This is preferable to throwing it away since it is
difficult to dispose of properly. Dow-Corning recommends incineration.
Notes:
1) Color: The DoT-5 certification requires that Silicone Brake Fluid be
purple. The dye that is used is only required to be stable enough to stay
purple until you pour it out of the original container. It turns out that
the purple dye is more soluble in DoT-3/4 fluids, so Dot-5 that has been
flushed throught a system full of DoT-3/4 will have lost its color to the
DoT-3/4 and the Silicone Brake Fluid will now be clear or slightly amber.
Something also happens to this purple dye after it has been installed in a
car for a year or more; that fluid will also be clear. I have a small vial
containing DoT-5 and Girling LMA that has been mixed for about 15 years; the
silicone is clear and the once amber LMA is now green.
2) DoT-3/4 fluids: These are classified as poly-glycols. They are a
mixture of compounds all chemically very similar to Ethylene Glycol, better
known as antifreeze. These compounds have a sweet taste, but are poisonous.
They should not be left out where your pets can get to them. Every year a
few dogs die after drinking brake fluid or antifreeze. I believe the lethal
dose for a person is about 100 cc.
3) Material compatibility: Brake fluids must be carefully chosen to be
compatable with all the materials exposed to them in the system. British
cars were once notorious for using natural rubber seals that required
special brake fluids. It is my understanding that by 1970 no one was still
making brake components or rebuild kits still containing natural rubber
parts. If you have a British car that still has 30 year old brake rubber
parts in it, then you MAY need to be concerned. DoT-5 Silicone Brake Fluid,
however, will be fine. It is compatable with all the elastomer materials
ever used in hydraulic brakes.
Jim Adney, copyright 1997
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Melissa Kepner Jim Adney
jadney@vwtype3.org jadney@vwtype3.org
Laura Kepner-Adney
Madison, Wisconsin
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