In this month's Skinned Knuckles (Sept 1993, vol 18, no 2), there is an
article about silicone brake fluid. Since this topic has come up a
number of times here, I thought I would summarize some of the points
covered in the article. The article is actually a summary of a number
of articles that have appeared in SK since 1980. The article is too
long to post in its entirety to the whole list (especially since I don't
have a scanner!) but if you would like to see the whole thing, let me
know and I'll see what I can do about getting you a copy. So, without
further ado: [my comments will appear in brackets like these]
Silicone Brake Fluid, A [summarized] Review. by Bill Cannon, Technical Editor
This article is a review of nearly all information we have published on
this topic since 1980. Most commentaries have been favorable to the use
of DOT 5 silicone brake fluid, but there have been a few detractors,
some of whom have complained of "leakage," "fluid expansion," and "fluid
compressibility." Few of the specific complaints have been capable of
documentation, and at the present time I feel that the use of DOT 5 has
been predominantly successful. Most of the problems that have been
described are thought to be due to improper use of DOT 5 or improper
rebuilding of brake components.
DOT 5 is not a panacea for a brake system that is defective, nor will it
correct corrosion that has progressed to the point of incipient failure
of the brake system. Simply bleeding out the old DOT 3 (glycol) fluid
and replacing it with DOT 5 (as some have proposed) will not correct
leakage or reverse corrosion that may have reached an advanced state.
The best time to use DOT 5 is when the brake system is being completely
rebuilt. Under these circumstances, DOT 5 will almost surely contribute
to prolonged life of the system, and the evidence for this seems
incontrovertible.
[A history of hydraulic brakes and brake fluids skipped]
[A description of DOT type fluids and their functional requirements skipped]
Table 1 Key requirements for DOT type fluids:
Requirement DOT 3 DOT 4 DOT 5
Dry boiling point (min) 401 F 446 F 500 F
Wet boiling point (min) 284 F 311 F 356 F
-40F Viscosity (max.) 1500 cS 1800 cS 900 cS
[Handling precautions skipped]
Outfitting your vehicle with silicone brake fluid - reprint of
procedures recommended by Dow Chemical, producer of DOT 5 fluid. (Note
instructions on avoiding entrainment of air bubbles and that Dow does
not recommend the flush/fill method of converting to DOT 5 fluid.)
Maximum performance of Dow Corning Silicone Brake Fluid is best attained
in a new rebuilt, clean brake system. Dow Corning Silicone Brake Fluid
is compatible with SBR, EP, Neoprene, and natural rubber. There are two
procedures for outfitting a vehicle. The first and recommended
procedure involves reconditioning, including the replacement of worn or
corroded parts. The second, a flush/fill procedure is applicable only
for relatively new, corrosion-free braking systems. This procedure will
give significant, but not complete corrosion protection and excellent
low temperature performance. High temperature performance will depend
on the amount of glycol remaining in the system.
[Directions for rebuilding brake system skipped]
Bleeding - Fill the master cylinder carefully, pouring the fluid down
the side of the reservoirs to minimize air entrainment. Let stand until
completely free of air bubbles. Bleed master cylinder if equipped with
bleeder valves. Do not shake fluid containers. Avoid agitating the
system while bleeding. Do not pump the pedal. Instead, depress and
release slowly. Perform bleeding cycle twice to ensure that all air has
been removed.
Silicone brake fluid field tests - [performed by Dow Corning]
Direct Mixing - Fifty brake systems were deliberately contaminated with
silicone fluid. In each case, the master cylinder was emptied and
topped off with silicone fluid, simulating and accidental mixing in
service. These fifty vehicles were operated for thirty months without
any incident of brake failure. (Editor's Note: Direct mixing is not a
recommended procedure. Glycol and silicone fluids are not miscible.)
Flush/Fill tests - In spite of careful efforts to completely flush
systems, subsequent teardowns revealed only partial success. From 10 to
30 percent of the original glycol fluid remained in the systems, even
though bleeding was continued until only clear silicone emerged from the
bleed ports. In spite of the failure to completely flush out old fluid,
there were benefits of introducing silicone fluid, including reduced
corrosion and improved low temperature pedal response because of the
lower viscosity of silicones at low temperature. One of the vehicles
put on test with the flush/fill method was a 1972 Olds Toronado. At the
time of the filling, engineers did not fully appreciate the limitations
of the flush/fill method in completely removing the residual fluid. The
Toronado was run for six years and a total of 93,000 miles. Its brake
system was then dismantled, at which time it was found that
approximately 30 percent of the fluid in the system was old glycol
fluid. Nevertheless, the teardown revealed no ill effects. All rubber
parts were in excellent condition. Wear and scuffing of pistons were
minimal. Some sludge from the original glycol fluid was noted, and some
corrosion of metal parts was found in direct proportion to the amount of
residual fluid at the site.
Pure silicone tests - (involved complete disassembly of all brake system
components, cleaning of wheel cylinders and calipers and lines, and
replacement of all seals.) One of the vehicles tested, a 1970 Chevelle,
was driven 55,000 miles over a two year period. On teardown, system
wear and corrosion were found to be non-existent. Water content of the
silicone fluid was less than 0.01 percent and the physical properties of
the silicone fluid were unchanged.
Conclusions of researchers -
1) Mixtures of silicone and glycol are possible with no adverse
chemical or physical effects.
2) Silicone flush/fills reduce corrosion and wear and improve brake
performance. However, this technique will not give the equivalent of a
pure silicone system.
3) Full benefits of silicone fluids come only with original fills
after complete removal of all residual glycol fluid.
US Army tests - In tests conducted under severe tropical conditions in
Panama, brake systems of US Army test vehicles using silicone brake
fluid performed far superior to systems equipped with conventional
glycol fluids. It was not uncommon for glycol fluids to pick up as much
as 10 percent water in a year under the extreme humid conditions.
Excessive rust from water pickup was the cause of many failures during
the test. In contrast, brake components from systems using silicone
fluid were relatively free of any corrosion, and water pickup was
virtually nil. Reference: U.S. Army Mobility Equipment Research and
Development Command, Report 2164, January, 1976.
[Author's personal experiences lauding the benefits of DOT 5 fluid and
bashing glycol fluids skipped]
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So there you have it. After reading this, you could think that there is
no reason not to convert to silicone fluids. It should be pointed out
that the focus of this magazine is the restoration and maintenance of
collector cars. For this application, it would seem that DOT 5 fluid is
a great way to go. From these reports, it also seems that it would be a
good choice for a car driven daily in normal driving. What is not
specifically addressed is how brake systems using DOT 5 fluid hold up in
more "spirited" driving. The author promises a second installment
consisting of reader experiences (pro and con) so maybe this will be
addressed.
Hope you found this worthwhile.
Mike
mburdick@unmc.edu
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