DOT 5

["Janet Scruggs" <scruggs@mbay.net>]

Dearest Group,  yes, I'm still quite far behind in my digest readings so if
this topic has been thoroughly covered or beat to death, please excuse the
redundancy.  

Comments regarding the use of DOT 5, silicone based, brake fluid have been
universally negative and offered without reason. Like every product we use
it has strengths and limitations and within it's boundaries the DOT 5 works
quite well.  I first used it about 1976 in my '67 Rover 2000TC in an effort
to improve the life of an otherwise maintenance intensive system.  DOT 5
worked great and, from my experience, _is_ compatible with DOT 4.  I
stopped using it due to it's expense and my returning to school.

The problem with DOT 5 centers around the base characteristics of silicone
fluid.  It is a relatively high viscosity fluid and therefore not usable in
our brake systems which are designed for a much 'thinner' fluid.  To
achieve a usable viscosity certain modifiers must be used... and therein
lies the problem.  These modifiers, I don't have a clue what they are, boil
at temperatures lower than what is encountered in high performance brake
systems.  What happens is under hard use the heat from pad/rotor reaches
the fluid in the caliper, the viscosity improvers approach their boiling
temperatures, get gassy (note the technical term) and expand, the brake
pedal goes soft and you have no brakes.  And a bad reputation is born.

Another way to describe high performance brake fluids, Super Blue, et al,
is 'high temperature' brake fluid.  

So then, for your high performance track/street car, any race car, and
probably your dad's heavy RV careening down a mountain... DOT 5 is not the
stuff to use.  For mom's grocery-getter, your commuter/beater, your garage
queen show car, or your Audi that is not 'enthusiastically' driven... the
DOT 5 works extremely well.  It is non-hygroscopic and lasts a long time...
perfect for the vast majority of applications.   

I did see Avi's comment re DOT 5 and the Corvette bleeding difficulty. 
Never owning one I must rely on the experience of a co-worker with a '71
454.  He doesn't use DOT 5 and is forever complaining about how hard his
brakes are to bleed.  Owners of those years Corvettes often use a 'gravity
bleed' technique where, in the absence of a power bleed system, you simply
let the thing sit for three days and the bubbles find their way up to the
reservoir.  Someone on the list, don't recall who, has a highly modified
Corvette of these years... comments?

Regards, Gross Scruggs
'87 5kCSq, Ate Super Blue