Re: hard shifting

[Robert Myers <rmyers@wvit.wvnet.edu>]

My apologies if this is a repost.  The first one bounced back to me from my
local machine.  We got prollums again.  :-(

At 08:23 PM 1/9/96 -0500, you wrote:
>Bob, 
>
>Water and Brake fluid don't mix, you should know that.  

Alcohols of four or more carbons are not completely miscible with water.
Therefore, they "don't mix" in all proportions.  They do, however dissolve
in each other in lower proportions at both ends of the scale.  Alcohol
(higher alcohols - not ethanol) based brake fluid will absorb water.  They
are not completely miscible (as water and the lower (1, 2, and 3 carbon
alcohols) but water will dissolve in them to an extent.  Water will dissolve
in typical 6 carbon alcohols to about 1 or 2%.  (I'm guessing at these
numbers.  They are approximately correct but I'd have to spend some time
with several references to give you exact numbers.)  This is not a huge
amount of water, but it can be enough to change (lower) the boiling point of
the mixture.  Atmospheric water (humidity) will be absorbed by alcohol based
brake fluid over time.  Water dissolves in alcohols due to the presence of
an -OH group in the alcohol.  This -OH group is what determines that the
material is an alcohol.  All alcohols have this group.  (Please forgive me
if I am beating a dead horse here.)  The -OH group of the alcoholic brake
fluid will form hydrogen bonds with the water.  This is what causes water to
be somewhat soluble in all alcohols.  This will tend to lead to a tendency
toward brake fade.  

A small amount of water in the brake fluid will lower the boiling point of
the mixture.  Upon heating under severe braking the water-contaminated brake
fluid can boil producing vapor in the system.  Vapor of any kind is
immensely compressible and when the brake is applied again this vapor will
simply be compressed and very little of the (power-assisted) pedal pressure
will be transmitted via the hydraulic system to the brakes.  Therefore the
fluid should be changed at intervals (every couple of years?) so that water
which has been absorbed is gotten rid of.

>If water is
>virtually incompressible why isn't it used in hydraulics like front end
>loaders?  They don't get hot enough to boil it away.  

Low temperatures are more of a problem as is corrosion of metallic parts of
the system.  It's rather hard to push ice through a rusted hydraulic system.
:-)

>The Castrol is more
>moisture resistant than Dot 3, LMA stands for Low Moisture Activity, its on
>the bottle.  By the way what exactly makes the fluid so dirty when it's old?
>Water.  And just to prove the point why do the brakes feel better with new
>fluid?  Its because the fluid acts as a liquid rod and there are no
>contaminates in the fluid to affect it's work.  If you still think I'm wrong
>put a little H2O in your master cylinder, pump it around the system and tell
>me the brakes aren't bothered by it's presence.

But they are bothered by the presence of water.  This is not caused,
however, by the (virtually non-existent) compressibility of the water.

>And you are correct when you state Dot 4 is not bothered by heat as Dot 3.
>That's because since the 4 doesn't absorb water the way 3 does.  Therefore
>when it gets hot there isn't as much water to boil, causing fade and
>possibly VAPOR LOCK!!!!!

True.  Higher boiling point alcohols are also higher carbon number alcohols.
The solubility of water in alcohols decreases with increasing carbon count.
The higher boiling point of the alcohol is what causes the fade resistant
quality of DOT 4 vs. DOT 3 fluids.  When the fluid is fresh there is
virtually no water present in it.  The water accumulates over time.

>Regards,
>
>Sam Strano
>Strano's Foreign Car
>strano@ncentral.com
>
>P.S.  That's why silicon (dot5) is good, although impractical.  It won't
>absorb water.   
>

Correct.  It also is much higher boiling and therefore doesn't boil as
easily as either DOT 3 or 4.

BTW, Sam, I don't think you are wrong in your conclusions, just the reasons
you gave for those conclusions.  I agree with you that water is bad for the
hydraulic system and that the system should be flushed and refilled with
fresh fluid periodically.

___
	Bob 
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*  Robert L. Myers                        <rmyers@wvit.wvnet.edu>  *
*  Chair, Department of Chemistry          304-442-3358 (office)   *
*  West Virginia Institute of Technology   304-574-2372 (home)     *
*  Montgomery, WV  25136                                           *
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