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Re: [T3] DOT3 v. DOT5

On 29 Jan 2005 at 20:26, James Montebello wrote:

> My apologies on the "mineral-based" error.  I've
> always heard them described this way, but looking this
> up after Jim's reply shows that "mineral oil" is or
> was used as a brake fluid in some applications, but
> "DOT3" and "DOT4" are typically not mineral oils, but
> glycol-based fluids. 

The "mineral-based" confusion appears on a number of web sites. I just try to 
shoot it down whenever possible. None of the offending web sites has ever 
choosen to correct this, however. AFAIK, there has never been a general purpose 
mineral=based brake fluid, but Citroen DID make a car (model, someone?) which 
used a dramatically different system which used a single fluid thruout the car 
for all hydraulic purposes. I assume, but don't know for sure, that this was a 
petroleum based hydraulic oil.

That's the only such case that I'm aware of.

> As for mixing of DOT5 and DOT3, I've seen this happen several times,
> and the result is usually many small, white-ish waxy globules.  I
> found this quote which may explain why this happens: "If silicone is
> introduced into an older brake system, the silicone will latch unto
> the sludge generated by gradual component deterioration and create a
> gelatin like goop which will attract more crud and eventually plug up
> metering orifices or cause pistons to stick."  So, perhaps this isn't
> caused by fluid incompatibility at all, but silicone incompatibility
> with this "sludge". 

Sludge is certainly present in amazing quantities in some systems. Mostly this 
appears to just be tiny particles of rust from cylinder bores and rubber worn 
off of the edges of master cylinder seals. There may also be residue from 
special lubricants applied to the brake components by the component 
manufacturers, some of which may well be silicone compounds.

When I was doing more of this work, and swapping more cars to DoT-5, I would 
occasionally come across some flushing residue from a car that would turn 
dramatically white if I added ethanol to the mix. I don't know what the 
reaction was, but it was something in the standard fluid which was doing this, 
not the silicone. This was something I came across while recycling the old 
fluid to recover the DoT-5, but it only seemed to happen in the early days, and 
in other people's cars where I had no way of knowing what brand of fluid was in 

My advice has always been to get as much of the old fluid out of the system as 
possible, simply to avoid the problems that it can cause, but in most of the 
cars which I've done, I simply pumped the silicone in behind the polyglycol in 
such a way that the old fluid was almost completely displaced. The 72-3 VW 
brake systems were expecially nice for this, because their calipers had bleed 
valves on both the top and bottom, so I could bleed old fluid out the bottom 
and finish by bleeding any tramp air off the top.

It was also nice that I could check a week, month, or year later, to see if any 
significant amount of old fluid had found its way to the bottom. I was always 
amazed to find that there was never more than one tiny bubble, usually less 
than 5mm dia.

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

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