What is the consensus on changing brake fluid?

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Brake Handboob by Fred Puhn.


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.

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.


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.


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.


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.

This book is available from Classics Motorbooks: www.motorbooks.com

Richard Lestage
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240SX 1989