<VV> Subject: Re: Compressibility of DOT-5 (Silione based brake fluid)

Hugo Miller hugo at aruncoaches.co.uk
Mon Jan 28 15:40:28 EST 2019

And of course there's Citroen in France that used to use mineral oil in 
their unique braking system (used under license by Rolls Royce at one 
time also I believe, along with Citroen's hydraulic suspension).
I mentioned in an earlier post that my (British) Morris Minor with 
Lockheed brakes maintains 2-3 PSI in the system at all times. This seems 
a sensible thing to do, as it keeps the seal lips splayed out, and I 
guess it solves the issue of water ingress via hoses etc. It also makes 
them a doddle to bleed.
There seems to be a sort of consensus emerging that silicone fluid 
works fine in a completely new system, but can cause problems if you 
just pump out the old glycol stuff & replace it with silicone fluid.

On 2019-01-28 15:02, Jim Simpson wrote:
> Hugo, yes you've got it about right.  DOT 3, DOT 4 and DOT 5.1 are
> all glycol based.  Each successive formulation has a higher boiling
> point making it better for heavy duty use such as in racing or
> mountain driving.  Most newer cars use DOT 4.  (DOT 2 was a mineral
> oil and pre-DOT fluids included alcohol and castor oil.  I'm not sure
> if there was ever a DOT 1 or if the first DOT brake fluid was just
> "DOT".  In any case, I doubt you could find any today.)
> All three (DOt 3, 4, 5.1) are hydroscopic, that is, they absorb
> water.  That doesn't have to be liquid water, it can be water vapor
> from the air.  In fact, water vapor will diffuse through the rubber
> hoses and seals of a brake system and be absorbed by glycol based
> brake fluids.  So just sitting there, the brake system is absorbing
> water from the air.  The absorbed water has two major effects; first,
> it lowers the boiling point of the brake fluid and second, it 
> promotes
> corrosion of the internal parts of the brake system.  Just about all
> vehicle manufacturers recommend changing the brake fluid every two
> years because of the moisture problem.
> DOT 5 was developed in response to the moisture problem.  It uses a
> silicone based fluid which is hydrophobic -- it repels moisture, even
> moisture that diffuses through the rubber components.  (In effect, it
> waterproofs the rubber.)  As long as the brake system was dry when it
> was initially filled, no water should ever appear in the brake
> system.  About the only way water could get in there would be a leak
> on the cap the master cylinder and rain or water from engine washing
> got in.  Hence there is rarely any need to change DOT 5 brake fluid
> which makes it very good for vehicles that are stored for long 
> periods
> such as classic cars and military vehicles.
> The major downside to DOT 5 is that it is not compatible with
> antilock brake systems.  And that's due to it's tendency to pick up
> air bubbles (foam) when agitated.  (You have to be very careful when
> filling a brake system with DOT 5 brake fluid so as not to mix in any
> air.)
> There are lots of claims that DOT 5 fluids are more compressible than
> the glycol based DOT 3, 4 & 5.1 fluids.  I can't find much hard data
> that would support that.  All fluids are compressible to some degree,
> but the range for what we would consider normal fluids is about 3 to 
> 1
> with water and oils on the low side and alcohols on the more
> compressible side.  Glycols seem to be more or less in the middle and
> silicone-based fluids perhaps on the high side of middle, but it
> depends upon the exact compounds used.  (Water would be great from
> compressibility standpoint and it would certainly be cheap.  But it
> freezes when cold, boils when hot and promotes corrosion. Liquid
> mercury is about 1/10th as compressible as water, but it also has 
> some
> drawbacks as a hydraulic fluid.)  
> The bottom line on compressibility though is that the compression of
> the brake fluid is largely masked by the flexure of the brake
> components such as the rubber brake hoses and bending of brake
> calipers.  And if there is any air in the system, that will have an
> even greater impact.
> I use DOT 5 in my Corvair along with steel braided flex hoses and
> all-new/freshly rebuilt components.  The brakes are firm and there
> have been no issues with leaks or corrosion.  The only problems I've
> had in the 20+ years since I changed over have been in the brake
> adjustments.  The grease (Lubriplate for brakes) in the
> self-adjusters up front dried out and kept them from adjusting
> properly.  Since I cleaned them up and used a synthetic grease, I
> haven't had any more issues.
> Jim Simpson
> Group Corvair

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