[Vwdiesel] Brake Fluid, '91 Jetta

[James Hansen]

DOT 3 or 4 are glycol based fluids. DOT 5 is silicone based.
4 has a higher boiling point than 3, it came out with the advent of "higher
performance" brake systems, such as metal impregnated disc pads.
Realistically, the ordinary passenger car should never see high enough brake
temps to worry about it, unless you live in the steep hills and ride the
brakes on long downhills without downshifting.  Advantage of 3 and 4 is it
turns color when it has too much moisture in it- moisture being that it
compresses more easily, metal parts can start to rust, and the boiling point
goes way down, to like half of where it started. Moisture is from the
ambient air.
Dot 5 is mostly silicone, boils at a way higher temp, but is finicky. It's a
race car thing, designed to have race car type maintenance... like every
time you drive it really.  It is much less of a lubricant, pistons tend to
stick when sitting over time, there is more master cylinder wear, but for
high temps and punished brakes, it is the balm, as it doesn't absorb much
water to lower the boiling point, and you can run red rotors all day and not
have pedal fade.
There is some dot 5 compatible glycol on the market now too, so it gets
muddled from there.
Story is, if it's turned brown, flush the system, there is water in it, and
if left there will cause grief. 3 is for valley dwellers, 4 is for the high
country, and valley dwellers with girlfriends in the hills. 5 is for racing.
Or so it goes...
-james



> -----Original Message-----
> From: vwdiesel-bounces at vwfans.com [mailto:vwdiesel-bounces at vwfans.com]
> On Behalf Of William J Toensing
> Sent: Monday, August 03, 2009 11:14 PM
> To: vwdiesel at vwfans.com
> Subject: [Vwdiesel] Brake Fluid, '91 Jetta
> 
> Can someone out there tell me what the difference is between Dot 3 &
> Dot 4 brake fluid I always thought Dot 4 was a higher quality brake
> fluid that was compatible with Dot 3 & was for use in heavy duty or
> more severe operating conditions. Dot 5 is silicone brake fluid. I like
> it because it is non-hydroscopic, that is it will prevent rust in a
> brake system. I am using Dot 5 in many of my collector cars. I rebuilt
> the brake system in my '41 Ford over 20 years ago & it still is working
> fine. However a mechanic friend of mine doesn't like it. I understand
> VW recommends against Dot 5. Does anyone know why? I have not put any
> Dot 5 fluid in any of my VWs or my modern cars. My understanding is for
> Dot 5 to work fine & prevent rust you have to use all new rubber
> components in order to prevent rust since Dot 3 & 4 will absorbed into
> the rubber components  & eventually absorbed water, a concern with
> collector cars that sit idle most of the time.
> Bill Toensing, Nevada City, CA
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