Chris Kotting wrote:
>
> I have seen this conversation before. Some people sing silicone's praises
> (myself among them), others relate horror stories. I don't get it. What's
> the difference? Any chance we can correlate the finer details (like who
> made the caliper seals and/or the brake fluid) and maybe determine what's
> causing this difference in experience?
>
> Chris Kotting
----
Very good questions, Chris. I get involved in it a lot because of my
job. There are some old answers, some vague answers and some solid ones.
When silicone fluid first came out, it was pretty pure. Most users had
Christopher Palmer's experience, and he's right about the cause. Brake
parts engineers have since the late 1920's counted on glycol's softening
and expanding effect to increase the size of the rubber 10 to 15
percent. Early silicones didn't do this. (I wonder if Christopher may
have gotten some old fluid.) The formula was modified with agents
intended to duplicate this effect. It took some time to get it
right--but there are still many knowledgeable people who are convinced
the silicone fluid does not have the proper effect on the rubber. This
seems to be mostly related to European brake parts, IME.
There are other, more significant problems w/silicone. It turns into a
very abrasive ash when it is burned. If it is used in a car with a
vacuum brake booster, and gets past the booster seals into the engine,
it can ruin the engine in *very* short order. It
is not so big a problem with later setups where the booster and master
are sealed separately--many of them even have drains between the master
and booster. It gets worse with the treadlevac type cylinders that have
vacuum against the secondary cup or seal, where the fluid is at ambient
pressure, and gets really bad in Midland-type boosters and the primary
master on Jaguars of the late fifties through seventies, where fluid
under pressure is present on one side of the seal in the reaction port
and vacuum is on the other side. You sure don't want that stuff getting
into a Jag V-12!!!
Any water that gets into DOT 5 fluid (and there are *several* ways this
happens, even though DOT 5 is not hygroscopic) will aggregate into drops
rather than mixing, and the specific gravity of water is greater than
the brake fluid. So it tends to settle down into low points. If you get
a drop of water in a disc brake caliper, and then get the caliper up to
212 degrees (not difficult--discs run hot anyway) the water will flash
to steam and the brakes on the front (or the entire car if a single
circuit master) are *gone*. Get a drop of water in a small port anywhere
in the system, experience freezing temperatures and all the brakes
beyond that point are *gone*. I believe this is the main reason OEMs do
not use silicone--a simple liability question.
Dot 5 fluid is often associated with a low and/or spongy brake pedal.
The pedal problem arises because DOT 5 tends to entrain air in
microscopic bubbles throughout the fluid. The labels on the fluid warn
against shaking and overly vigorous bench bleeding because of this
characteristic. Since the air does not aggregate, it can't be bled out,
and it often causes a soft pedal. Most of my customers who have switched
back to DOT 3 say this is the reason. And when I set up at the Reno
(Nevada) Swap Meet, I always get several people from sea level asking if
I know why they have lost most of their pedal--the pedal goes almost to
the floor. I ask if they have silicone fluid, and guess what the answer
is. The ambient air pressure at Reno's elevation is enough less than at
sea level that the air in the fluid expands, making the pedal problem
even worse. One person I have communicated with on this subject says he
knows many who have undertaken Pikes Peak tours who have lost their
brakes 2/3rds of the way up and had to be towed down!
Other points about DOT 5--to the best of my knowledge, *NO* OEM has ever
put DOT 5 fluid in a standard production car at the factory. This is a
very telling argument for me. Manufacturers of even the most expensive
cars, to whom price would be no object, install and specify DOT 3 or DOT
4. If anyone knows otherwise, I would appreciate hearing about the make,
model and year. (I'm aware that some fleet purchasers such as the
military and the USPS have been known to specify DOT 5 in the bidding
documents, but these are not standard production vehicles.) It is
corrosive to electrical contacts, as in brake light switches--German
cars seem to suffer most from switch failure. Again to the best of my
knowledge, *ALL* makers of ABS specifically prohibit the use of DOT 5.
Most of the fleet managers I do work for who had converted their fleets
to DOT 5 have converted them back to glycol, and most of the
professional restorers I do work for recommend against it--including, as
Christopher mentioned, Norman and David Nock of British Car Specialists,
who are two of the most knowledgeable Brit car people on the West coast.
In a show car, that is driven little or not at all, it makes a lot of
sense. But IMO any car used regularly enough to be called a "driver"
should have glycol fluid. BTW, whether the cylinder has been sleeved has
no bearing on this. A properly sleeved cylinder will not leak air at
brake pressures, much less silicone. A sleever who tells you not to use
DOT 5 because it might leak around the sleeve is telling you that he
didn't put the sleeve in properly. Most sleevers do recommend against
the use of DOT 5, but it is for the reasons above and their
recommendation applies to all systems, sleeved or not.
Joe
--
==brake cylinders sleeved with brass==
==One-Wire conversions and rebuild
kits for GM alternators 1963-85==
==Quincy, CA==
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