william sadler <wsadler@copper.ucs.indiana.edu> writes:
> Since I have to rebuild my master cylinder I thought I would switch
> from DOT4 to DOT5 brake fuid in the interest of saving my paint.
>
> Do I really have to replace every single wheel cylinder in the car?
> Or can I just flush the system with DOT5 several times?
Here's the official Party Line, as forwarded to me a while back from
someone's FAQ list (I think in the rec.autos newsgroup):
<begin_quote>
Q: How often should I replace my brake fluid?
A: Probably more often than you do. Traditional brake fluids tend to
absorb water; this water can corrode internal parts of the brake
system and also lower the boiling point of the fluid. DOT-3 type
fluids are least sensitive to boiling point problems, and are common
in older Detroit iron where brake fluid is generally ignored for 10+
years by zero maintenence owners. While this practice is not
recommended, it can be gotten away with. DOT-4 type fluids offer
_much_ improved brake pedal feel, but absorb water more readily and
must be changed more often; their performance when wet degrades more
rapidly than DOT-3 type fluids. Once a year is recommended for DOT-4
fluids, although agressive drivers may profit by changing out fluid
more frequently, or at least bleeding a modest amount of fluid out of
the brake calipers fairly regularly. DOT-5 (e.g., silicone) fluids
are another matter, which is addressed in the next article.
Q: What about Silicone (DOT-5) brake fluids?
A: Silicone fluids are a tricky proposition. Unlike DOT-3 and DOT-4
fluids, they do not absorb water at all. While this may sound like
a feature, the problem is that any water present pools up in such
systems, interfering with braking performance and corroding any metals
at the spot where the pooling is occuring. The water will tend to
migrate downwards in the braking system to the brake calipers, where
most of the corrosion occurs.
Because of this phenomenon, it is essential when converting to
Silicone to empty the entire brake system and flush it throughly
beforehand; some even recommend replacing all rubber parts in the
brake system when converting to DOT-5 fluids.
Two other issues that come up with silicone fluids: 1) they are
difficult to pour cleanly (that is, without air bubbles), which
interferes with getting a good brake pedal feel, and 2) while they
generally have much higher boiling points than DOT-4 fluids, they
do have high temperature failure modes which are indistinguishable
in effect from boiling DOT-4 fluids. DOT-5 fluids may make sense
in some street car applications, but they are certainly not
recommended for high performance driving applications, and the
economics are questionable for street use.
<end_quote>
Andrew C. Green
Datalogics, Inc. Internet: acg@dlogics.com
441 W. Huron UUCP: ..!uunet!dlogics!acg
Chicago, IL 60610 FAX: (312) 266-4473
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