What is the consensus on changing brake fluid?
Brake Handboob by Fred Puhn.
BRAKE-FLUID SPECIFICATION
Dot-3 and dot-4 fluid are glycol based while Dot-5 is silicone based.
DOT fluid must meet 2 specifications. A dry boiling point and a wet boiling point. The dry
boiling point is for fresh fluid from a new container while wet boiling point is the same
fluid that have absorb moisture.
DOT 3 DOT 4 DOT 5
Dry boiling point (deg F) 401 446 500
Wet boiling point (deg F) 284 311 356
Note that fluid boiling point drops after it is exposed to water. DOT 4 fluid meet DOT 3
requirement. You may buy in store 500F DOT 3 fluid and 450F DOT 4. The temperature shown
on the containers are always dry boiling point. The 500F DOT 3 meet the dry boiling point
requirement of DOT 4 but is not DOT 4 because it does not meet the DOT-4 wet boiling
point.
For a racer which bleed is brakes before his race, a 500F DOT 3 is better than a 450F DOT
4. But for a long term use, the DOT 4 while be better after a few month.
SILICONE BASED DOT 5 FLUID
If you want the highest wet boiling point and a different combination of properties, you
might considerer changing to silicone-based brake fluid. This fluid does not absorb
moisture like a glycol-based fluid. Therefore, its wet boiling point is much higher. Also,
silicone-based brake fluid do not damage the pain on your car if spilled.
However, for racing, use only a glycol-based racing brake fluid. Silicone fluid has been
tried in racing, but it has a tendancy to give a spongy pedal after exposure to high
temperatures. This is due to the slight compressibility of silicone brake fluid at high
temperature. For ordinary street driving, this is not critical, but a racer needs all the
brake-system stiffness he can get.
SWITCHING TO SILICONE
If you change to silicone brake fluid, you must first clean all of the old fluid from the
brake system to get the maximum benefit of the silicone-based fluid.
If you merely bleed out and install new fluid, you will have a mixture of the two fluids.
This will work, but it won't be as good as it could be.
ABOUT DAMAGING YOUR SYSTEM
This occurs commonly when U.S.-manufactured brake fluid is used in an older foreign car
with natural-rubber seals. The seals swell and have to be replaced eventually.
A good application for silicone brake fluid is in antique and collector cars. Brakes in
these types of cars are never subjected to high-temperatures. Protection of the internal
parts from corrosion is most important. Silicone brake fluid serves this purpose
beautifully, because the absence of moisture in the system pratically eliminates the
chance of dreaded internal corrosion.
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This book is available from Classics Motorbooks: www.motorbooks.com
Richard Lestage
[email protected]
240SX 1989