REFLECTOR: brakes fluid

[sbjknox]

> From: "Chris Barber" <cbarber at texasattorney.net>

> Subject: Re: REFLECTOR: brakes fluid


> I thought it was due to one type being needed for the automotive master
> cylinder (Toyota ??) and another fluid being Needed for the aviation brake
> cylinders. Dot 5 can be used in both.

> On Mar 9, 2011, at 5:55 PM, "Kirk Aragon" wrote:
> Barry,
> I believe that DOT 3 is more hygroscopic than DOT 5 and that it is 
> possible
> for DOT 3 fluid to absorb enough water that it will actually freeze in the 
> brake
> lines while the aircraft is at altitude. If it doesnt have time to thaw 
> during
> descent, it might present the pilot with significant issues on touchdown.
> Kirk

Yes, DOT3 is hygroscopic and DOT5 (Si) is not.  However, that is not the 
whole picture...
Any automotive or aircraft hydraulic brake system, over time,  will collect 
water.  DOT3 is formulated to keep the water in suspension and contains 
additives to attempt to mitigate the effects of the water.  But after awhile 
(several years), the fluid becomes so 'nasty' (water, microscopic dirt and 
the chemical brew that results) that it begins to attack the brake 
components and really needs to be changed.  Most people trade automobiles 
often enough that the contaminated fluid does not become an issue.
DOT5 is not hygroscopic and the water that enters the system remains 
separated and settles to the bottom of cylinders and just sits there slowly 
rusting (assuming steel cylinders) the cylinders and pistons.  Bleeding 
generally won't remove the water as it sits on the bottom, away from the 
bleed valves.

I have a '57 Triumph TR3 that I have owned since '59.  In the '60's I 
decided to go with DOT5 and have stayed with it.  It has steel slave 
cylinders and whenever I open one up, I can see the corrosion on the bottom 
where the water sits.  I use DOT5 in my Velocity and Starduster.  I use DOT3 
in my other autos.
What would I use in a 'new' application?  Probably DOT3.  Why?  because once 
you have DOT5 in a system, it is a pain in the butt to adequately flush it 
well enough to change to DOT3.  And I believe a quality, modern DOT3 will 
have the additives to deal with the moisture  issue.

Those of us with old cars and airplanes are well served to drain and flush 
the brake systems periodically (10 years ? ) regardless of the fluid type.

$0.02

Sid Knox
Oklahoma

Velocity  173 RG  N199RS
Starduster  N666SK
KR2         N24TC
W7QJQ