Brake Fluid Specifications and Testing

Mark B · Dec 2, 2007

Following on from this thread I have found this Defence Standard for the requirements and testing of DOT 4 brake fluid (obviously based heavily on SAE J1703).

It clarifies that what is normally referred to as the "wet boiling point" is the the boiling point of the fluid when it contains 3% by volume of water.

It also gives the minimum high and maximum low temperature viscosities.

Should I ever find the same info for DOT 5.1 fluid I will post it.

Mark B · Dec 2, 2007

This document gives the major specification requirements for DOT 3, 4 and 5.1 brake fluids. The numbers quoted below are:-

dry boiling point (Celsius), wet b.p. (Celsius), maximum viscosity at -40C (centiStokes)

DOT 3, 205, 140, 1500
DOT 4, 230, 155, 1800
DOT 5.1, 260, 180, 900

The reduction in low temperature viscosity required by DOT 5.1 is necessary to ensure that systems such as ABS and ESP can still function properly in winter, when you tend to need them most. It is also interesting to note that the commonly used DOT 4 fluid is actually worse than DOT 3 in this property. Most very high boiling point fluids, common sold as "racing brake fluid" cannot meet the low temperature viscosity requirement of DOT 5.1 and so only meet the DOT 4 spec, despite their high boiling points.

Brake Fluid Specifications and Testing

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