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

Jim Simpson simpson661 at gmail.com
Mon Jan 28 15:02:12 EST 2019

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

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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