Jim,
I think it's a little different than that ... conventional (glycol-based)
brake fluids are hygroscopic, as you note, and will absorb moisture like
crazy. This drastically lowers the boiling point, which will allow the
fluid to vaporize at a lower temperature & cause the "vapor brake" scenario
you describe. So glycol-based fluids must be changed often, particularly in
high-humidity areas.
Synthetic fluids don't absorb moisture. So as long as you keep the system
filled, moisture cannot get into the sytem at all. Essentially the
synthetic fluid surface forms a barrier and will not let moisture in.
HOWEVER ... the dry boiling point of synthetic is much lower than racing
glycol-based fluids, so the synthetic fluid can boil all by itself.
My recollection is that synthetic fluids have dry boiling point around 350 -
400 deg F. It never changes, since it doesn't absorb moisture. Regular DOT
glycol (mineral) based fluids may be a bit higher, and the racing stuff like
AP can be bought with dry boiling points of 550 - 600 F. BUT, and this is a
big but, the boiling point of a glycol-based fluid (even AP 600) goes way,
way down as it absorbs moisture -- like 300 deg F or lower. So if it isn't
changed regularly, it falls to a LOWER boiling point than synthetic.
That said, I have used synthetic in my racing MGA (4 wheel drum brakes) for
years, and never boiled the fluid. I suspect that this is partially because
drum brakes don't transfer as much heat to the fluid, as disc brakes would.
In a disc brake you have a very large heat transfer area through the pad &
piston, a the fluid volume is pretty close to the heat source. The heat
transfer path in a drum brake (lining, to metal part of shoe, down to point
contact on cylinder, through relatively small diameter piston) is much less
"efficient". So I suspect that drum brakes can tolerate lower boiling point
of fluid.
Net, I'd be very cautious about using synthetic in a disc brake application,
but in my case of all drums (and skinny VSCCA tires which limit braking
ability) it has worked just fine.
Regards,
Mark Palmer
MGA #185
>From: Jim Hayes <hayes@mediaone.net>
>Reply-To: Jim Hayes <hayes@mediaone.net>
>To: "Richard E. Buckingham, Jr." <rebjrmd@ix.netcom.com>
>CC: Carl McLelland <carlmcle@saturnnet.com>, Vintage list
><vintage-race@autox.team.net>, Alpine list <alpines@autox.team.net>
>Subject: Re: Synthetic Brake Fluid
>Date: Fri, 24 Dec 1999 16:34:11 -0500
>
>As it was explained to me: Silicone-based brake fluids do not absorb
>moisture. That's great if there is none in them, or if you have a car
>that gets easy useage. But a race car gets fluid very hot. Regular brake
>fluid is hygroscopic - it absorbs moisture which lowers its boiling
>point slightly. Silicone fluid keeps the moisture separate, which means
>that if it gets very hot, the moisture may turn to steam and you are
>dealing with not hydralic but "vapor brakes" which don;t stop too well!
>
>Castrol LMA, AP or the other racing rated fluids are the choice for
>racing and change every spring or more often.
>
>Jim
>
>"Richard E. Buckingham, Jr." wrote:
> >
> > I am unfamiliar with ATE Blue Racing brake fluid, but if it is Silicone,
>I
> > have some experience.
>--
>Jim Hayes Winchester, MA, USA
>hayes@mediaone.net http://www.fotec.com/jim/jim.htm
>jeh@fotec.com http://www.fotec.com/
>All generalizations, with the possible exception of this one, are false!
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