The Real Story: Conventional vs Silicone Brake Fluid
Today's current
Lockheed "Universal", Girling "LMA", and silicone brake fluids are so
vastly superior to the old Girling "Green" and "Crimson", and Lockheed
"Heavy Duty" fluids originally specified for most of our older British
sports cars, that it would make no sense to use the older types today,
even if they were still available. The most notable advances have been
in raising boiling points, improved compatibility with each other, and
reducing moisture absorption.
The main function of brake fluid
is to transmit pedal movement to the brake pads and shoes. To do this
efficiently, brake fluids must be non-compressible. They must also not
boil at the highest operating temperatures encountered, thicken or
freeze at cold temperatures, not corrode or chemically react with any
materials in the hydraulic system, and not decompose or form sludge,
gum, or varnish at any temperature. They must lubricate internal moving
parts, flow easily through small passages, have a long and stable shelf
life, and be compatible with other brake fluids.
Brake fluids
are classified by their chemical type and boiling points. The different
chemical bases currently used are polyalkylene glycol ether (commonly
called glycol), silicone, and mineral oil. (Of these, mineral oil
doesn't concern us, as it is used in very few cars, none of which Moss
Motors deals with.) D.O.T. 3 and D.O.T. 4 brake fluids are
glycol-based, while silicone-based fluids are classified as D.O.T. 5.
To further confuse matters, there is now a D.O.T. 5.1 brake fluid which
has a diethylene glycol-ester base, with properties similar to D.O.T.
4, but with enhanced performance characteristics. These D.O.T.
(Department of Transportation) specifications also indicate minimum
boiling points. It is important to note that these D.O.T.
specifications are performance specifications, not material
specifications; for example, D.O.T. 5 sold in Europe is not
silicone-based, as it is in the USA.
In the good old days,
little good could be said of brake systems. Warnings such as, "as the
cups in the master cylinder are pure rubber; it is imperative to use
only the recommended fluid. Any other fluid may be dangerous" were
common. Such strong concerns were very valid in the 1950s, much less so
now, even for 1950's vintage cars. The reasons for this lessened worry
about our hydraulic systems "turning to goo" if the wrong fluid is used
is that: 1.) pure rubber hydraulic seals are no longer made for our
cars, and 2.) D.O.T. 3, 4, and 5 brake fluids are safe to mix, and are
compatible with the seals now available. While these brake fluids are
safe to mix, mixing them is not recommended.
When brakes are
applied on a moving car, the kinetic energy of the car is turned into
heat. The faster the car is moving and the faster it is stopped, the
more heat is produced. Some of this heat soaks into the brake fluid. In
the late 1940s, brake fluid with a boiling point of 235° F was
considered adequate. By about 1957, the lowest S.A.E. specification was
for a minimum boiling point of 302° F for cars with drum brakes.
Disc
brakes presented new problems. In stopping faster (and often heavier)
cars more quickly, they generated even more heat which had to be
dissipated, with an accompanying requirement for brake fluid with even
higher minimum boiling points. Improvements in brake lining materials,
brake drum and rotor design and metallurgy have also had a similar
effect; improvements in braking efficiency require improvements in
brake fluids. To handle these higher temperatures, improvements were
also made in wheel cylinder and brake caliper seal design and
materials.
Brake fluids must not be allowed to boil for two reasons:
1) The brakes won't work due to the vapor bubbles being compressible.
2)
Physical and chemical properties of the brake fluid may change due to
the "lighter" components boiling off. Glycol-based brake fluids in
particular, are hydroscopic (moisture absorbing), some more so than
others. When water is absorbed, the boiling point is sharply lowered.
This occurs because water boils at only 212° F. When brake fluid is
mixed with water, the boiling point of the mixture is less than that of
the "dry" brake fluid. See chart for D.O.T. minimum boiling point
specifications.
| D.O.T. Minimum Boiling Points (degrees Fahrenheit): |
| | D.O.T. 3 | D.O.T. 4 | D.O.T. 5 |
| Dry | 401 | 446 | 500 |
| Wet | 284 | 311 | 356 |
*This
is the required min. specification, but does not reflect actual
performance of silicone-based fluids. Since D.O.T. 5 is
non-hydroscopic, its actual "wet" boiling point is essentially the same
as its dry boiling point. However, a brake system using silicon-based
fluid with water present in the system would show an effective boiling
point of 212 degrees due to the free water.
Water contamination
also leads to corrosion of brake pipes, wheel cylinders, calipers, and
master cylinders, resulting in pipe leaks, "frozen" cylinder pistons,
accelerated seal wear, and the formation of sludge. Silicone fluids
avoid these problems by being non-hydroscopic (not moisture-absorbing),
while glycol fluids can absorb as much as 6% water just by being in a
"sealed" automotive hydraulic system for a few years. This moisture is
generally absorbed from the air. Some moisture even works its way into
brake hoses. Most comes from master cylinder cap vents and resultant
condensation in the air space above the fluid, and from allowing cans
of brake fluid and master cylinders to remain open to the atmosphere
for too long. Silicone fluids absorb a tiny amount of moisture (on the
order of 280 parts per million, or .0028%) and then absorb no more.
Silicone
fluids, in addition to having high boiling points and being
non-hydroscopic, do not damage paint as do glycol fluids. This is of
particular importance in regard to show cars where a spill or leak of
glycol fluid can have seriously ugly results. There are, however, some
disadvantages to silicone fluids. They are slightly compressible,
particularly near the higher end of their temperature range. While this
is of absolutely no consequence for normal street use, this is why
silicone fluids are not used in race cars. (Conversely, racing
hydraulic fluids should not be used in street cars. This is because,
although racing brake fluids have high dry boiling points, most are
highly hydroscopic, and have relatively very low wet boiling points.
They would probably work extremely well if you were to change the fluid
every week or so.) Because air bubbles do not regularly dissipate in
silicone brake fluid, special care must be used to prevent them from
forming during pouring and bleeding operations. The best way to bleed a
silicone fluid system is with an Eezibleed Kit (Moss #386-860). Lacking
that, bleed with slow pedal strokes, avoiding "pumping" the pedal. It
may be necessary to bleed the system again in a day or so if there were
any air bubbles which wouldn't bleed out the first time.
A
newly rebuilt and scrupulously clean brake system filled with silicone
fluid should outlast a system filled with glycol fluid by several
times. There is little advantage in adding silicone fluid to a system
which contains even small amounts of contaminants. Merely bleeding the
system is not enough, as there will be pockets of old fluid and sludge
which will not bleed out. Silicone fluid tends to concentrate any
residual glycol fluid, moisture and sludge, into slugs, instead of
allowing their dispersal throughout the fluid, as does glycol fluid.
This can lead to relatively severe but localized problems, rather than
the more general system deterioration experienced with old
moisture-laden glycol fluids. This may be a factor in reports of
leakage when silicone fluid is used in non-rebuilt systems which had
been used with glycol fluid. A "new" system full of silicone fluid will
require very little maintenance for years.
Old dirty
moisture-laden brake fluid is hazardous; it can't be relied upon to
stop your car reliably. It is a little known fact that glycol brake
fluids must be changed regularly, much as engine oil must be changed.
The Austin-Healey 100-6 and 3000 Workshop Manuals specify brake fluid
changes every 18 months or 24,000 miles (whichever comes sooner), and
examination of all fluid seals and hoses in the hydraulic system, with
replacement as required, every 3 years or 40,000 miles. Other
manufacturers had similar recommendations. While silicone fluid change
intervals may be safely extended, do not overlook periodic checks,
especially of hoses. Please take care of your brake system for your own
and other's safety.
Working on automobiles is inherently dangerous. Moss Motors, Ltd. is not liable for injury or damage due to incorrect installation or use of their products. All products are sold with the understanding that the safe and proper installation and use of the products is the customer’s responsibility. Follow factory workshop manual procedures and instructions, but use current shop safety standards and common sense. Some tasks will require professional advice or services which Moss Motors cannot provide.If you have a specific comment or question and you'd like an immediate reply from Moss Tech Services, don't post here - please first email: BritishTechnicalSupport@mossmotors.com Please include the Title and Location of this tip if applicable.
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