Brake Fluid Overview

Current DOT ratings for brake fluid is DOT 3, 4, 5 and DOT 5.1. DOT 5 is silicone based and all the others are glycol based. The DOT 5 silicone based brake fluid is not compatible with the any other brake fluids.

Brake fluids are made up of mainly to different type bases.

Glycol based which is Polyalkylene Glycol Ether
Silicone based

Brake Fluid

The DOT ratings
The table below shows the MINIMUM wet and dry boiling points for DOT 2, 3, 4, and 5 brake fluid in degrees Fahrenheit.

NOTE:The DOT 2 spec is for drum brakes and is obsolete. If you have any DOT 2 , do not use it. DOT 5 is a silicone brake fluid. Silicone brake fluid (DOT 5) should be avoided because it is not compatible with regular brake fluid.

DOT 3 and DOT 4 fluids are compatible with each other and may be interchanged or mixed with no ill effects.

	DOT 2	DOT 3	DOT 4	DOT 5
Dry Boiling point	374	401	446	500
Wet boiling point		284	311	356

The DOT specifications are based on the concept of wet and dry boiling points. The dry boiling point is applicable when fluid is fresh and the wet boiling point after the fluid has been exposed to moisture and has had the opportunity to adsorb water. The minimum values for the wet and dry boiling points are specified for each DOT level, and increase from 3 through 5.

Below are some popular brake fluid specs.
Note:The "Estimated BP After 6-months" column is estimated based on the principle that brake fluid gains about 3.5% moisture per year, which is where the wet boiling point is measured.

Fluid	Dry BP	Wet BP	Est. BP after 6 months	US $/oz
Castrol SRF	590	518	554	$2.076
NEO Super DOT	585	421	503	0.983
Motul Racing 600	585	421	503	0.712
ATE Type 200/ATE Super Blue	536	392	464	0.295
Valvoline High Perf Synpower	513	333	423	0.16
Castrol LMA	450	311	381	0.219
DOT 5 Spec	500	346	423	N/A
DOT 4 Spec	446	311	379	N/A
DOT 3 Spec	401	284	343	N/A

Why do we worry about the Wet Boiling point?
The boiling point decreases over time and can create vapor by boiling in the caliper. The result can be a sudden brake failure. Also the water in the brake fluid can contribute to corrosion of parts.

I normally use the ATE brake fluid and alternate between the Gold and the Blue. The different colors make it easy to tell when you have completely flushed the old fluid out. It normally runs around $10.00 a liter and I change it every other year.
ATE Super Blue has a dry boiling point of 536F and a wet boiling point of 396F. It is compatible and will mix with most DOT3, DOT4, or DOT 5.1 Fluids.

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