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Re: [T3] DOT3 v. DOT5


I'd like to chime in with my 2 cents worth. Unfortunately I have to disagree 
with some of Jim's comments. Here's what I believe to be true.

On 28 Jan 2005 at 21:48, James Montebello wrote:

> DOT3 and DOT4 are mineral-based brake fluids.  DOT5 is
> pretty much always silicone-based.  

"Mineral based" generally means petroleum based, which would not be true. DoT-3 
& 4 are hydrocarbons, like petroleum, but not compatable with petroleum. In 
fact, petroleum products (mineral oils) are harmful contaminants in any brake 
system. DoT 3 & 4 are polyglycol fluids which are hydrocarbons (so they could 
be called organic) rather closely related to ethylene glycol, the main 
component of anti freeze. DoT 3 & 4 will not mix with mineral oils, but they 
mix freely with water, just like anti-freeze.

DoT-5 is alsways silicone based.

> Mineral-based fluids are relatively thin, and are hydroscopic.  They
> like to attract and retain water. Over time (months to years, depending
> on climate), enough water will collect in these fluids to lower their
> performance (they boil at lower temps, and boiling fluids compress
> easily, so they lose their ability to transmit force from the master
> cylinder to the slave cylinders effectively).  All that water also
> tends to cause lots of internal rust on any iron or steel brake
> components (the British particularly are fond of cast-iron in brake
> parts).  They're also very nasty to paint.  Get a drop of mineral
> brake fluid on paint it and will eat it up very quickly. 

A semantic point, but it's not that boiling fluids compress easily, but rather 
that the vapor that results from the boiling is easily compressible.  

> Silicone-based brake fluids are relatively thick, and not hydroscopic. 
> They're not destructive to paint. Because they don't attract water,
> they don't lose their effectiveness nearly so quickly, and they don't
> promote rust in brake parts. 

I don't find a significant viscosity difference between the  DoT-5 that I use 
and the DoT 3 that is commonly available. I DO have some extremely thick 
silicone fluids here, but the DoT-5 brake fluid was compounded to be a good 
replacement for DoT 3 and 4.  

As far as viscosity is concerned, one of the benefits to me has been the fact 
that the DoT 5 did NOT increase in viscosity in very cold weather the same as 
normal fluids do. I certainly agree that a much thicker fluid makes a poor 
brake fluid.  

> Sounds all rosy for silicone, yes?  There's a catch, of course. 
> Silicone, being fairly thick, aerates very easily.  There are nearly
> always many small bubbles in the fluid.  Getting ALL of the air out of
> the system is very, very hard.  Air is very bad in fluid as it expands
> greatly under heat.  It also compresses very easily.  So, while DOT4
> will eventually lose its effectiveness, the anti-DOT5 types insist that
> DOT5 doesn't work well out of the box (er, bottle). 

I don't see this problem in practice. There IS apparently a problem with 
dissolved air coming out of solution and giving rise to very small bubbles, but 
this goes away with time and, frankly, I've never noticed any effect. You have 
to keep in mind that, regarding bubbles, the only thing that matters is the 
total volume of bubbles in your system, and 1 cc of bubbles amounts to about 
1/2 mm of piston travel in your master cylinder. I suspect that it would be 
hard to have 1cc of bubbles in your system (we're talking here about bubbles 
that are too small to be seen, but lots of them.)

> Oh, and if there are any plans to switch, mixing mineral and silicone
> fluids results in a goo that's nearly impossible to remove, and will
> happily block the small passages in many brake systems.  You need to
> use lots of brake cleaner and flush out the braking system, or use all
> new parts, when switching from one fluid to another. 

I've heard this stated many times, but I have no idea what the basis of it 
might be. Above my desk at work I have a little bottle that consists of 1/2 
Girling LMA DoT 4 polyglycol brake fluid and 1/2 Dow-Corning DoT-5 silicone 
brake fluid. The two fluids have coexisted in that same bottle since 1978, so 
it's up to 27 years. I shake them up occasionally, but they just separate again 
with time. The only thing that has changed is that the color dye in the 
silicone has all moved to the polyglycol, and this happened the very first time 
I shook them up together.

I'm not sure what's in brake cleaner. I would be VERY reluctant to use it on 
brake rubber parts or inside anything where I did not have a way to let it 
thoroughly dry out. Most solvents are just way too hard on brake rubber parts. 
I stick to water and ethanol.  

I've been using DoT-5 since 1978. I have probably installed DoT-5 in 70 
different cars, mostly imports. I freely admit to being a DoT-5 advocate. I've 
had a few problems with brake systems with DoT 5 in them, but I don't think the 
brake fluid was at fault in any of them. 

I have a more complete silicone FAC which is available to anyone who is 
interested. Just send me an email and I'll forward you an electronic copy.

-- 
*******************************
Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
*******************************

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