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Brake Fluid Comparisons

This information was compiled by Gary Hackney and was edited for Volvo cars by S. Ringlee. Many of the racing fluids with poor wet boiling points or fluid incompatibilities were removed since these fluids are not designed for everyday use. Only Glycol (Polyalkylene Glycol Ether) fluids are shown; no silicone DOT 5 fluids are listed because they may not be used in Volvo cars without major brake system modifications. The new DOT 5.1 glycol fluids may be used, since they are compatible with DOT 3 and 4.

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. See the graph below right for wet boiling points (DOT 4=blue) Note that these are minimum values, and there is no constraint on by how much a manufacturer may exceed them. To achieve a DOT rating, the fluid must meet both dry and wet boiling point specifications. In addition to DOT level, another important consideration in selecting a brake fluid is the presence of anti-corrosion additives, commonly found in street fluids but not in track fluids. Note that Volvo ABS-equipped cars must have brake fluids flushed every two years to remove moisture, renew additives, and prevent corrosion.

Corrosion Issues. [Adapted from Brake and Front End magazine, May 2004] The single most important reason brake fluid must be changed regularly is to replenish the anticorrosion additives. Corrosion inhibitors, pH stabilizers and antioxidants are added to brake fluid to improve the long-term corrosion protection of brake systems. Over time these corrosion inhibitors can become depleted leaving the internal parts of the brake system vulnerable to corrosion. There are many variables involved in determining how long it takes to deplete the corrosion inhibitors including brake fluid chemistry, chemical and thermal stability, brake system design, driving habits of the operator, frequency of maintenance, temperature, and road surfaces. Another unrelated extensive study found that the buffer capacity and inhibitor concentrations "drop to less than 10% of their initial levels after only 30 months of service". (Jackson, SAE paper 971007,Corrosion Prevention SP-1265, 1997) The rate of depletion is affected by many factors. One of the studies found the rate of depletion is fastest at the wheels. This is where the fluid is exposed to the highest degree of heat and the heat causes the corrosion inhibitors to breakdown. Vehicles with ABS show even faster degradation due to the aggressive circulation of the fluid caused by the cycling of the ABS system. This, combined with the fact that ABS systems use close tolerance valves and other precision parts, makes them more susceptible to the affects of corrosion or deposits.

Copper has a direct role in the corrosion of the brake system, as well as providing an indirect relationship to the age of the brake fluid. In a NIST report, Ricker et al hypothesize "the copper in the brake lines corrodes at a slow rate over several months or years resulting in copper ions in the brake fluid. These ions then act as oxidizers and plate out in the ABS valves when the corrosion inhibitors can no longer prevent corrosion of the ferrous components. According to this hypothesis, copper corrosion starts when the vehicle is new and proceeds at a rate that is limited by the oxidizer content of the brake fluid, mass transport of this oxidizer, and the effectiveness of the corrosion inhibitors in the brake fluid at retarding copper corrosion." Copper is the first or "Alpha Contaminate" and will corrode before other metals in brake system according to Ricker because "even though copper is in galvanic contact with more active metals, the low conductivity of the brake fluid allows copper corrosion to proceed." You might ask how does copper get in the brake fluid? The answer is from the brake lines. The inside surface of the brake lines is coated with a copper brazing alloy.

How do you accurately determine the copper content of the brake fluid? The answer comes in the form of test strips that provide a way to determine the "virtual age" of brake fluid . The patented FASCAR® technology used provides a measure of the copper in the brake fluid which indirectly provides a measure of the level of corrosion inhibitors in the system. The test is simple and straightforward. Simply dip the strip in the brake fluid of the reservoir for one second. In 30 to 120 seconds, the reaction zone will change colors depending on the condition of the brake fluid. Compare the color of the reaction zone and make the appropriate recommendation. Replace the brake fluid if copper content exceeds 200 ppm.

In the absence of the test strips, just flush according to the manufacturer's recommendations.

Shelf Life. [Tip from Castrol RE: LMA Brake Fluid] Shelf life is no longer than 2 years, provided the oil has been stored in a cool, dry area, with no extreme fluctuating temperatures and the seal has remained intact.

[From Hackney:] The "Estimated BP After 6-months" column is estimated based on a page I found that says brake fluid gains about 3.5% moisture per year, which is where the wet boiling point is measured. [Editor: See the chart to the right from BMW, which notes an approximate gain of 2.5% per year]. Assuming linear degradation this column is where you'd be. The last two columns may be confusing. They're the price per ounce, divided by the degrees F the fluid exceeds the dry or wet DOT spec; sort of a price for performance number where lower is better. Yeah, I'm an engineer. FWIW: My conclusion: ATE Type 200 and Super Blue are not only the cheapest of the performance brake fluids, they are also the cheapest per degree of boil protection, and have a very high 6-month BP estimate. In addition you can alternate with each change and the color difference will tell you when you're done. [Note that ATE Blue is not DOT-approved because of the color.] ATE and some of the higher-performance fluids are available from IPD and FCPGroton. Valvoline and Castrol are commonly found in mass merchandisers.

Conversions °F = (C x 1.8) + 32
  °C = (F - 32) x .5555

Fluid Dry BP Wet BP Est. BP after 6 months US $/oz US $/oz per °F > DOT4 DRY US $/oz per °F > DOT4 WET
Castrol SRF 590 518 554 $2.076 $.0144 $.0100
NEO Super DOT 585 421 503 0.983 .0071 .0089
Motul Racing 600 585 421 503 0.712 .0051 .0065
Motul DOT 5.1 509 365 437 0.675 .0107 .0125
ATE Type 200/ATE Super Blue 536 392 464 0.295 .0033 .0036
Valvoline High Perf Synpower 503 343 423 0.16 .0023 .0073
ATE SL 500 329 415      
Castrol LMA 450 311 381 0.219 .0547 n/a
Gunk DOT 4 Brake Fluid 510 311 411 0.16 .0024  
DOT 5 Spec 500 346 423 n/a n/a n/a
DOT 4 Spec 446 311 379 n/a n/a n/a
DOT 3 Spec 401 284 343 n/a n/a n/a

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