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NOTE:The DOT 2 spec is for drum brakes and is obsolete.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 4DOT 5
Dry Boiling point 374401446500
Wet boiling point 284311356

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.

FluidDry BPWet BPEst. BP after 6 monthsUS $/oz
Castrol SRF590518554$2.076
NEO Super DOT5854215030.983
Motul Racing 6005854215030.712
ATE Type 200/ATE Super Blue5363924640.295
Valvoline High Perf Synpower5133334230.16
Castrol LMA4503113810.219
DOT 5 Spec500346423N/A
DOT 4 Spec446311379N/A
DOT 3 Spec401284343N/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.

Low Viscosity Brake Fluid

Recently I was getting ready to order some ATE brake fluid and noticed on their website they had a new brake fluid that was optimized for ABS braking systems.

Their new fluid is SL.6 and these are the benefits as listed on their website

Advantages of Original ATE SL.6

  • Low viscosity (thin-bodied)
  • Permits fastest response by ESP
  • Optimum brake fluid for electronic brake systems like ESP, ABS, TCS, etc.
  • Improved handling safety owing to speedy response of ESP
  • Most vehicle manufacturers already use the low-viscosity DOT 4, Class 6 (ISO 4925) brake fluids in production cars (OEM and OES)

The nw SL.6 has a dry boiling point of 509°F (265°C) and a wet boiling point of 338°F (170°C)

While the dry and wet boiling points are only about 5% lower than their racing fluid, the trade off is a faster operating ABS system.

Below is the graph ATE uses to demonstate the advantages of the low viscosity SL.6

The other trade off is ATE recommends changing the SL.6 every two years where as their racing brake fluid was recomended to change every three years.

Seeing as I am not on the track, I would probably benefit from the quicker responding ABS system.

I also found another low viscosity brake fluid with a good following, Pentosin.

Interesting, the dry and wet boiling temperatures are identical to the ATE SL.6.
The Pentosin is available locally so I am going to give that a try and see how it does.

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