Re: DOT 5 Brake Fliud

[Mike Orth <ORTHM@xxxxxxx>]

Jerry asks about 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.
>
This is true.  Another drawback is that since DOT 5 forces water to
collect, corrosion also starts faster at these points. Add to point 3.


> 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.
>
True again, especially when it gets hot.  This can also make the ABS
pump work harder.  Not good considering  that point number 4 is also
valid.  ABS pumps are $$$$.  While I have not personally driven a car
with DOT 5 in it, the people I've talked with that have said that the
pedal is spongy most of the time.  Feels like a DOT 3 or 4 system that
needs bleeding. Hummm, not a good trait for us track junkies.


> 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.
>
Yes again, but fully draining the system is not enough.  The system must
be flushed out as well.  The time to convert is when the entire brake
system is being rebuilt.


> 4) Lubrication.  DOT 5 apparently has no lubricative qualities.
>
ABS pumps are not happy campers about this.....


> 5) BMW blessing.  BMW does not recommend DOT 5 in it's braking systems for
> the above reasons.
>
See points 1 - 4.

The main use of DOT 5 that I have seen has been in restored cars that
don't get driven around much.  They spend most of their time in a
garage.  In this situation,  the fluid's resistance to absorbing water
is an advantage.

Stick with a good DOT 4 and change it every year on a street car.

Mike Orth
Tarheel Chapter

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