On Mon, 1 Jul 1996, Will Zehring wrote:
> >Some lurkers ouSome lurkers out there might
> >flame me for not recommending DOT 5 (silicone) fluid for MGs, as this
> >is the trendy way to go in the 90s, but I didn't want to go into all
Actually, it was my impression that that great determiner of automotive
fashion, the U.S. military, was in the process of phasing the stuff out,
and most likely this would signal the end of the trend.
> >the problems we've had with brake light switch failure and discs
> >locking on when we tried to use silicone fluid.
>
> I don't doubt Paul's statement, from his experiences. Is this the
> experience of others as well? I use silicon fluid because I've been told it
> is superior to Dot4 in terms of dryness, and it is also not a paint
> stripper, like Dot4 is, but it takes as good a care of rubber as Dot4 does.
Word on the street is, DOT5 is probably OK for normal use, but changing
over from DOT4 can cause trouble. In our case, after changing to DOT5 the
brake hoses (all six of them) swelled shut, making driving the car a very
interesting experience. I guess this sort of thing doesn't happen in
every case, but it happens often enough to be a trend. Apparently this
is not a problem if you replace every piece of rubber in the brake system
at the same time you change over.
My two more serious objections to DOT5 are related to its behavior in use.
Apparently silicone is a little teeny bit more compressible than propylene
glycol, and as such it may be suspect in hard use. My other concern is
that since DOT5 fluid and water are immiscible, if any H2O finds its way
into the system (not inconceivable, you must admit), it will stay all by
itself in a little pocket somewhere in the lines. Here is the biggest
problem: in addition to the strange local rusting that such a water pocket
can cause, it radically lowers the boiling point of the brake system. Dry
DOT4 boils somewhere in the 400's F. Wet DOT4 boils somewhere in the
300's F. Dry DOT5 boils somewhere in the 500's F. So far so good, but
wet DOT5 (i.e. a water pocket somewhere in the line) boils somewhere in
the 200's F. This is why racers won't touch the stuff, I'm told.
Oh, and I heard somewhere that DOT5 fluid actually will damage paint,
although it takes a good deal longer than DOT3 or DOT4 to do it. So don't
be too complacent if it spills.
Michael Chaffee
mchaffee@staff.uiuc.edu
[Note new address]
CCSO probably thinks I'm working right now. What do you suppose they think
I'm saying?
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