Regarding DOT 5

["Scruggs, Gross" <GScruggs@monterey.nps.navy.mil>]

	>Date: Tue, 13 Apr 1999 09:25:10 -0500
	>From:	Luis Marques <marques@raytheon.com
<mailto:marques@raytheon.com> >
	>Subject:	Re: DOT 5

	>"Greg Herrmann" <ai7rtech@isoc.net <mailto:ai7rtech@isoc.net> >
wrote:
		>>
		>> I am considering changing over to DOT 5 brake fluid.
What do I need
		>> to do for the conversion?
		>>
		>> Greg Herrmann
		>> 84 CGT
		>> 86 CGT
		>> 93 NSX
	>
	>
	>In a word, "don't".  DOT5 is not compatible with DOT4 and you will
have
	>to completely flush all the system (if not completely rebuild it).
	>These are mostly silicone-based fluids that don't absorb water, and
are
	>intended for museum cars that don't get driven but maybe once a
year.
	>The fluid itself is more compressible than DOT4 (spongy pedal), and
	>since it doesn't absorb moisture, the little moisture that makes
its way
	>into the system stays undiluted causing much more rapid and severe
	>localized corrosion.  Plus there have been tales of brakes staying
	>locked up like if you have the parking brake on...  in the middle
of the
	>road after a light turns from red to green... not fun.

(snip)

While I agree with Luis' advice re switching to DOT 5 brake fluid there are
some other issues.  DOT 5 is _chemically_ compatible with DOT 4... but it is
not _functionally_ compatible with DOT 4.  
Nothing vile will happen if you mix the two.  I switched to silicon brake
fluids in the mid-70s, before there was even a DOT 5 classification, with
nothing other than topping the reservoir and bleeding until the new fluid
comes out, and ran that way for years with no problems.  (Show of hand,
please, of all who have actually flushed a brake system?  Why did feel it
necessary and are you really going to admit to putting, what... alcohol?...
in your hydraulic system?)  At the very least you must remember the ultra
conservative nature of any US Government organization, in this case the
Department of Transportation.  They are not going to approve two
incompatible fluids for use in your brake system given the certainty that
some 'common denominator' out there will mix the two and cause a situation
for which they may become liable.  They are chemically compatible.  
Let's review what we know about DOT 5:  It is a silicone based fluid and in
it's 'natural' state has high viscosity and is too thick to readily replace
the fluid in your brake system.  So they have to add viscosity improvers to
make it behave in ways that make it compatible with the dynamics of our
systems.  Silicone fluid is not compressible... but when the very high heat
of high performance braking is passed to the DOT 5 fluid the viscosity
improvers become gassy and expand, giving you not only a spongy pedal at the
end of the back straight on the fifth lap but that 'brown in the pants'
feeling as well.   
On the very positive side, however, DOT 5 will not absorb water..
non-hygroscopic is the traditional term, although Bob Myers tells me that
use of 'non-hydroscopic' is seeing increased usage.  By not absorbing
moisture it is an ideal fluid for brake systems which are either seldom used
or never stressed by high heat.  Your cherished '66 Corvair Monza that
resides mostly under the car cover in your garage and my Mom's minivan that
is driven only very gently are both perfect candidates for DOT 5.  In fact
the Toyota that you won't admit to using as your commuter is also perfect
for DOT 5 as it is not likely to see much preventative maintenance or track
time... at least mine didn't.  
However... if the vehicle under consideration is going to see either track
time or pull a trailer through the mountains... don't use DOT 5 as it is not
made for those high temperature situations.  Lots of folks on the qlist are
not into performance events and for the conditions under which they drive
DOT 5 is a valid answer.  One screaming panic stop won't tax the heat
absorbing capabilities of DOT 5, but many will.  
And another thing, the more I see and experience the more I understand that
life is indeed best described by the parable of the blind men each relating
their impression of an elephant.  Each of us is describing what we feel is
self-evident truth but is actually only a part of something greater.  I
welcome the relating of any experiences out there that can either validate
or improve upon my understanding of this elephant.  Remember, we are all
here to teach and to learn.  We don't have to be civil on the roads, but
perhaps we should agree to be civil in this forum.  After all... the
serenity of knowing that that most of you are assholes and are just waiting
to pounce on anything I say frees me from the need to say so.  That... and
the knowledge that owning an Audi requires an advanced sense of humor...
peace be with you.
Regards, Gross Scruggs
'87 5kCSq, many mods... ATE Super Blue