Originally Posted By: tommygunn
@Ken2, FMVSS116 *requires* brake fluid to be compatible with components used in brake systems.
S5.1.10 Compatibility.
(a) At low temperature. When brake fluid is tested according to S6.10.3(a), the test specimen shall show no sludging, sedimentation, or crystallization. In addition, fluids, except DOT 5 SBBF, shall show no stratification.
(b) At 60 °C. ( 140 °F. ) . When brake fluid is tested according to S6.10.3(b)—
(1) Sedimentation shall not exceed 0.05 percent by volume after centrifuging; and
(2) Fluids, except DOT 5 SBBF, shall show no stratification.
S5.1.12 Effects on (styrene-butadiene rubber wheel cylinder) cups. When brake cups are subjected to brake fluid in accordance with S6.12—
(a) The increase in the diameter of the base of the cups shall be not less than 0.15 mm. (0.006 inch) or more than 1.40 mm. (0.055 inch);
(b) The decrease in hardness of the cups shall be not more than 10 IRHD at 70 °C. (158 °F.) or more than 15 IRHD at 120 °C. (248 °F.), and there shall be no increase in hardness of the cups; and
(c) The cups shall show no disintegration as evidenced by stickiness, blisters, or sloughing.
S5.1.13 Stroking properties. When brake fluid is tested according to S6.13—
(a) Metal parts of the test system shall show no pitting or etching to an extent discernible without magnification;
(b) The change in diameter of any cylinder or piston shall not exceed 0.13 mm. (0.005 inch);
(c) The average decrease in hardness of seven of the eight cups tested (six wheel cylinder and one master cylinder primary) shall not exceed 15 IRHD. Not more than one of the seven cups shall have a decrease in hardness greater than 17 IRHD;
(d) None of the eight cups shall be in an unsatisfactory operating condition as evidenced by stickiness, scuffing, blisters, cracking, chipping, or other change in shape from its original appearance;
(e) None of the eight cups shall show an increase in base diameter greater than 0.90 mm (0.035 inch);
(f) The average lip diameter set of the eight cups shall not be greater than 65 percent.
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By the way, FMVSS = Federal Motor Vehicle Safety Standard which includes all the U.S. regulations covering vehicle lighting, brakes, motorcycle helmets, and much, much more.