Re: My brake job advice & brake fluids <long>

[Jerry Chyo <jerryc@xxxxxxxxxxxx>]

Hey all,

A few days back I posted in response to someone's query on doing an E28
brake job.  In my response I advised changing out their brakes fluid with
DOT 5.  Now here is my dilemma.  Soon after my post I was advised that DOT
5 was the worst possible thing I could put into my car and that the advise
I had given was potentially catastrophic and maybe even life-threatening.
I queried a respected (at least by me) member of the Digest who has given
mounds of good info in the past if this was correct.  His advise was that
DOT 5 ISN'T bad.  I would like to get to the bottom of this because I am
due for another brake job and was planning on changing to SS lines and swap
to DOT 5.

Here are the particulars against DOT 5:

1) Water absorption.  DOT 4 absorbs water making it boil at a lower temp.
DOT 5 doesn't absorb water but water nonetheless makes it's way into our
system.  Since the DOT 5 doesn't absorb the water we now have 2 fluids in
our brake system making the non-absorption fact moot.

2) Compressibility.  DOT 5, from make to make, has differing
compressibility which may give uneven or inconsistent results.  I was told
of a story of a guy whose brakes locked SHUT and they blamed it on the fluid.

3) Corrosion.  It is standard practice to fully drain your hydraulic system
if you are going to replace your brake fluid with DOT 5.  Unfortunately you
apparently run a great risk of corrosion anywhere that the DOT 5 and DOT 4
or lower fluids interact.  This corrosion is apparently pretty bad.

4) Lubrication.  DOT 5 apparently has no lubricative qualities.

5) BMW blessing.  BMW does not recommend DOT 5 in it's braking systems for
the above reasons.

What does the collective wisdom think.  I have seen many folks advise the
use of DOT 5 in their braking system and have not noticed any dissenting
opinions until now.  I must admit that if true, these are very valid points
and I would lean towards using DOT 4 in my own car.  Let the debate begin.

Thanks for the ear.

Jerry Chyo
'88 M5
'72 tii

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