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

[Pat Donahue <pdonahue@xxxxxxxxx>]

Jerry writes:
> 
> 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.

I'll stick some comments in your remarks below.  The gist of what I have
to say is that DOT 5 fluid _is_not_ a good idea if the maker of the car
(BMW in this case) says not to use it.

> 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.

I suppose under extreme conditions one could get water into their brake
fluid, but the normal route is for warm brake fluid to absorb moisture
out of the air that moves in and out of the vented reservoir.  The
venting system does a pretty good job of excluding liquid water.  Since
DOT 5 doesn't absorb this moisture it'll pretty much stay in the air and
go back out the same way.  Even if you did get some water in it the
water would migrate to the brakes (or clutch slave) and you could easily
bleed it out without even releasing too much fluid.

> 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.

The compressibility issue is one that you can feel.  DOT 5 fluid is
prone to holding onto really tiny bubbles of air and there are some
precautions to be taken while bleeding this into a system.  I worry
about vibration going down the road and working air into the system.

As far as brakes locking shut I laughed when I heard about this.  I was
running Silicone (DOT 5) and had had no such problems.  Then it
happened.  Heading south on 395 out of DC in heavy traffic all lanes do
a quick panic stop.  This happens on a regular basis (but still catches
_way_ too many people by surprise).  Traffic starts rolling again, but
my front brakes won't release!  I rocked the car some and stomped the
brake pedal a couple of times and eventually got rolling though with
obvious front brake drag.  It cleared up as I drove so I ignored it
thinking it a fluke.  Weeks later I stopped normally at a light and
couldn't get the car to roll when the light went green.  Using much gas
and a couple hundred miles of back tires I got off the road into a
service station.  I opened the front brake line at the master cylinder,
got a spurt of Silicone out and I was on my way.  I don't know the hows
and whys of this but changed to DOT 4 and no more problems.

> 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.

Error.  The Silicone bottle will even tell you that the fluids can
coexist in your braking system but won't mix.

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

I don't know how much lubrication is needed, but see (2) above.

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

Plenty of reason to stay away by this point.

> 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

My disclaimer:  All of my experience was using Silicone in my MGB which
is 1972 technology.  I'm betting the results hold and won't be risking
it.
- -- 
Pat Donahue  CCA 125031
Vienna, VA
'91 M5 4GTFUL
'72 MGB 8LYFGR&

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