Brake fluid

[rich]

Brake fluid can come in a number of forms, standardized under the DOT (Department of Transportation) standard.

DOT 2 is essentially castor oil;
DOT 3, like DOT 4 and DOT 5.1, is a polyethylene glycol based fluid (contrasted with DOT 5 which is silicone based).

DOT 3, DOT 4 and DOT5.1 are hygroscopic and will absorb water from the atmosphere. This degrades the performance of the fluid by drastically reducing its boiling point. In a passenger car this is not much of an issue, but can be of serious concerns in racecars or motorcycles.
Most cars produced in the US use DOT 3.

DOT 5 is a silicone based brake fluid. It is not compatible with any polyethylene glycol based fluids.
Unlike polyethylene glycol based fluids it is not hygroscopic, however this means that instead of being dissolved, any water in the system will eventually find its way to the lowest point in the brake line, corroding it, and potentially boiling and causing brake failure. Because of this, and other reasons (such as compatibility, cost, and lack of advantage over regular fluids) it is no longer used very much.



Boiling Point Ranges
Dry Boiling Point -Wet Boiling Point
DOT 3 205°C - 140°C
DOT 4 230°C - 155°C
DOT 5 260°C - 180°C
DOT 5.1 270°C - 191°C

This entry is from Wikipedia.

[Clive]

Originally Posted by slogger

Thanks Miket. As I said, could be wrong. I have always used Castrol Universal. This very issue arose when I had total brake failure due to my E Type master. Did not want to damage my new one.

In the good old days (1960s) the British motor industry used two manufacturers of brake components, namely Lockheed and Girling. Both used different types of rubber for their seals and the brake fluids were incompatable, therefore you used Girling fluid in Girling brakes and Lockheed in Lockheed. If you put the wrong fluid in the wrong system as sometimes happened then brake failure was iminent. Castrol then brought out the Universal Brake Fluid which meant that one fluid could be used in either brake system without damage to the rubber seals. This has now become the norm. If you are contemplating Silicone brake fluid then the existing system will have to be COMPLETELY drained and cleaned as the two flluids (mineral and silicone) DO NOT MIX!