Volvo Maintenance Hints for 7xx/9xx - Bill Garland's Nuclear ...

Brake Fluid Comparison FAQ Home
Volvo Maintenance FAQ for 7xx/9xx/90 Cars Version 7.3
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. 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.
From BMW: Brake Fluid Moisture Effects
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