Hello List:
Given that brake fluid discussions are a perennial favourite, I have
condensed a message I wrote to another list touching upon some of the
superficial points of the various types available. It is by no means
exhaustive, but does address a few points raised in this ( and almost
every other ) brake fluid thread.
>> begin condensed message <<
First off, allow me to touch upon the generalities of brake fluids,
ratings, composition, and DOT ratings.
Currently, to my knowledge, there are three commercial formulations for
brake fluids; two are glycol based, and the third is silicone oil based.
The DOT 3 fluids are a mixture of polyalkylene glycol ether and other
glycols ( all start life as ethlyene glycol - antifreeze ); the DOT 4
fluids add borate esters to this mix. ( Compounded correctly, these
borate ester-enhanced glycol fluids can meet or exceed the DOT 5 spec. )
The DOT 3/4/5 rating is independent of the composition of the fluid, and
is a rating of relative performance. The rating refers in large part to
the wet and dry boiling points of the fluids: (all in degrees
Fahrenheit )
DOT 3 wet - minimum 284; dry - minimum 401
DOT 4 wet - minimum 311; dry - minimum 446
DOT 5 wet - minimum 356; dry - minimum 500
Unless a great deal has changed in the last two minutes or so, silicone
brake fluid is not really compatible with glycol-based( also called
polyether-based ) brake fluid. The don't mix, they handle heat
differently, silicone tends to be compressible under a wider range of
circumstances, and they handle water differently. Plus, silicone has an
affinity for the degraded bits and pieces of junk in the braking system,
and tends to form a sludge with them, eventually blocking passages and
impairing brake function if allowed to accumulate for too long.
Silicone brake fluid has a number of strengths, and a number of
drawbacks:
Strengths
1.) High boiling point - since it does not absorb water, there
is no so-called wet boiling point
2.) Doesn't absorb moisture
3.) Doesn't remove paint
4.) The viscosity is more stable over the extremes of
temperature
5.) With the exception of some formulations used in external
boots, silicone brake fluid is compatible with all current braking
components
Drawbacks
1. ) Very hard to pour without entraining air bubbles - hence an
application will generally have a softer, spongier pedal feel
2.) Doesn't absorb water - any water in the system accumulates
in the lowest point of the system and stays there, concentrating rust
3.) Whereas glycol fluids begin to compress near their boiling
points, silicone fluids begin to compress at around 300 to 350 degrees
Fahrenheit.
4.) Additives in the fluid can vaporize at comparatively
moderate temperature, increasing the spongy feel.
5.) Silicone fluids expand significantly when hot.
6.) Functionally incompatible with systems which have held
glycol-based fluids for any length of time, requiring flushing and seal
replacement( Note that there are counter opinions on this that state
that the modern silicone formulations are in fact compatible with only a
flushing, rather than a complete reseal. ) The actual DOT specification
requires chemical compatibility, so as far as that goes, the two fluids
won't cause reactions if used in the same system, but they certainly
won't mix, either.
7.) Functionally incompatible with anti-lock brakes. In the
first place, the silicone fluids tend to be more viscous, which can
cause problems with the timing of the pulses, which are intended to work
with the thinner glycol-base fluid. This sometimes leads to damage to
the ABS valving. Secondly, the rapid pulsing necessary to anti-lock
function tends to cause cavitation in the fluid, as the tiny bubbles
collapse and coalesce into larger ones, and then collapse and reform
into smaller ones. This tends to nullify the ABS effect, can diminish
the actual effective braking to a dangerously low level, heats the fluid
leading to further sponginess, and can damage the ABS controller.
Thirdly, silicone brake fluid tends to foam when expressed from a small
orifice under pressure. This, of course, reduces its hydraulic
effectiveness to nothing in the area affected.
8.) The silicone tends to attract and bind with the fragmentary
wear products of the rubber components in the brake system, creating a
gelatinous sludge, which can block fine passages, particularly in ABS
systems.
Having dispensed with these generalities, let us turn to Steve's
particular questions:
> <<I would suggest using DOT 5 silicone brake fluid. It is very high
> temperature (won't boil under extreme use), does NOT absorb
> water, and is
> completely compatible with DOT 3 or DOT 4 brake fluids.
( Grudgingly tolerant of, is more like it - ed. )
> It is best to completely replace the existing fluid with
> DOT 5 but not
> necessary.
This is patently not so, at least not for straight silicone brake fluid.
The two fluids are immiscible, and have different densities - one floats
on the other. There is some talk of a silicon ester-based fluid, with a
boiling point of 590 degrees Fahrenheit, which is miscible with
glycol-based fluids, but I don't think that any of these are
commercially available.
> DOT 5 is expensive compared to DOT 3 or DOT 4. But you can
> forget period
> flushing of lines, rust in lines, wheel/master cylinders, etc.>>
This, also, is both false and unwise - moisture is still absorbed into
the brake system through the seals at the same rate, only instead of
being held in suspension, accumulates in huge globules. This tends to
accelerate local corrosion, and should that large globule of water ever
boil, could easily dangerously compromise braking almost
instantaneously.
>
> There has been much discussion as to brake fluid, biennial
> changing of brake
> fluid, brake system fluid compatibility, etc.
>
> Is DOT 5 suitable for use in (our) more conventional MB's?
No.
> Is it compatible
> with the DOT 4 in them now?
No. Chemically, mixing the two will not create a third, explosive or
acidic or otherwise harmful compound, but functionally they do not get
along.
> Will it reduce maintenance as described in the
> last quoted paragraph?
No. One still must replace the fluid periodically to flush the
gelatinous rubber-dust slime and the occasional lurking water bolus.
> Has anybody used/switched to DOT 5
> in their cars,
> other than Mogs, and how did it work out.
I have read of the experiences of a number of members of my Triumph list
who have gone to silicone brake fluid - other than flushing and
resealing the system, and the seemingly permanently slightly spongy
pedal, most reported that they were satisfied. One very real issue with
these cars is that the seals are often not what they could be in the
brake and clutch master cylinders in the first place, and _every_ owner
has stories of leaking MC seals destroying the paint on the firewall, so
this aspect represents a very real value. Also, these are often weekend
cars, for pleasure driving only, so many don't see a lot of brake system
heat.
> Remember, a lot of the time, the maintenance instructions
> and schedules
> presented with the car represent the technology at the time
> that it was
> produced, and the introduction of new materials, chemicals
> and oils may render
> the original recommended service intervals obsolete. MB
> probably does know
> best about their products, but they print owners manuals
> for the car during
> the year of its production, and don't update or modify
> these schedules when
> new products are introduced.
I do not think that this is the case with brake fluids. Further,
Mercedes periodically updates their fluid specifications.
Having torn silicone fluid down, who uses silicone fluid then? The sole
major user, to my knowledge, is US Government, mainly the armed forces,
although the Post Office is a big user, too. They use it because it is
stable and predictable over a wider range of temperatures, has a high
boiling point, and doesn't absorb moisture. ( It does not appear whether
the government cares about the paint issue. ) I can find no evidence of
any other major benefits that the government expects from the silicone
fluid.
Stuart Steele
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