Hi all
I did a little research on this for my other hobbies (Corvairs and VW
Things). I'm not an expert, but I'll try to recall what I found out. The
purpose of brake fluid is to apply mechanical energy from a driver
controlled source (the master cylinder) to each of the road wheels through
a rather circuitous path of hydraulic lines. In order to do this
effectively the hydraulic lines have to be dimensionally stable so that
they don't expand when pressure is applied from the master cylinder and are
able to transmit as much energy as possible to the calipers and/or wheel
cylinders. These hydraulic lines and brake components are often fabricated
out of steel. Unfortunately steel tends to suffer from the effects of
corrosion. DOT 3 & 4 fluids are hygroscopic, meaning that they are
DESIGNED to absorb water, thereby preventing that water from pooling in the
lowest places in the hydraulic system. Where does this water come from?
At best there will always be water introduced due to diurnal heating and
cooling. This mixing action has the effect of both preventing internal
corrosion and raising the boiling point of the brake fluid to a value
higher than that of water(450 f for dry brake fluid vrs 212 f for water).
If fluid changes are performed regularly this works very well. However we
all know that some cars are not maintained as often as the manufacturer
would specify. This is where the use of silicon based brake fluid (dot 5)
becomes attractive to some operators. Dot 5 has some unique properties.
Among these are its noncorrosiveness and its inability to absorb water. It
also seems to exhibit a degree of compressibility that has the effect of
raising pedal effort for a given amount of braking, which explains why many
racers don't like to use it. Dot 5 is also immissable with Dot 3 or 4.
This means that it will not readily mix with these fluids. So if you wish
to convert a dot 3 system to dot 5 you must thoroughly clean the system in
order to completely purge the older fluid, least that older fluid collect
in the low spots in the system. This usually means a complete disassembly
of the system. What would happen if you simply drain the system and flush
it with dot 5? Well because these two fluids don't mix, you'd get areas in
the system where the old fluid collects, along with any moisture and
contaminates that it had. So, you'd have a compressible fluid that wasn't
effective at corrosion control. In short the WORST features of both types
of fluids! And remember that since there will always be diurnal heating
and cooling, there will always be moisture introduced into your dot 5 brake
system! Having said all this, I came to the conclusion that dot 5 was and
is the way to go for cars that aren't used very often. All three of my
Corvairs, and my Thing have dot 5 brake systems, and this spring I will
probably convert my Spitfire.
Hank
78 Spit
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