DOT 3 & DOT 4 are basically the same. DOT 4 has a higher boiling point.
DOT 4 is -NOT- silicone-based.....-ONLY- DOT 5.
DOT 5 is not new. Can't find when it came into being, but I remember seeing it
in the '70's.
DOT 5.1 is really just a higher boiling point super DOT 4 developed for the
racing community. Why they named it 5.1 instead of 4-point-something or 6 is
beyond me.
Several interesting articles:
<http://www.xs11.com/tips/maintenance/maint1.shtml>
<http://www.motorcycleproject.com/motorcycle/text/cows-brakefluid.html>
Plenty more to delight, educate, and amuse.....just Google...
Pete
-------------- Original message ----------------------
From: "Richard Schnoor" <machworx@gmail.com>
> Its been a while since I have worked on my roadsters, but I have some
> questions: Forever DOT3 was the standard brake fluid which is hygroscopic
> (absorbs water from high humidity air) which then ends up rusting your brake
> calipers. Some one told me about DOT 4 which was silicone based & was not
> hygroscopic so rusting wasn't a problem for a seldomly driven car. When I
> changed over to "4" my brakes were extremely spongy & was accordingly was a
> problem with this type of brake fluid. I even built a pressurized auto
> bleeder so I could bleed my brakes alone but resulted in no reduction in
> sponginess. So what is DOT 5 & 5.1? Do these new fluids address the
> sponginess or rusting problem?
>
> On 2/19/08, John F Sandhoff <sandhoff@csus.edu> wrote:
> >
> > Keep in mind that DOT5 has been reported to be harder to get a
> > clean system bleed with. Power bleeding seems to be more
> > effective than other methods. Also, DOT5 will find its way thru tiny
> > seal imperfections so a system that's leak-free on DOT3/4 may end
> > up leaking on DOT5.
> >
> > I don't know how DOT 5.1 behaves. It's apparently not silicone based
> > like 5.
> >
> > I personally use DOT4 - Castrol GT/LMA for years and years but have
> > switched to Valvoline synthetic DOT4.
> >
> > -- John
> > John F Sandhoff sandhoff@csus.edu Sacramento, CA
> > ________________________________________
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>
>
>
> --
> Richard Schnoor P.E.
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