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Dynamometer tests of a production disc brake provided new information on asbestos fiber emissions during break-in, normal use, and high temperature use conditions. Both ambient air and brake cooling air were sampled isokinetically, using 0.45 μm filters. Examination of test and background filters required a clarification process to maximize fiber detectability, the use of transmission electron microscopy (at 40,000X) for detection, and electron diffraction for positive identification of asbestos fibers. Most of the lining asbestos was found to be converted to a nonfibrous material by the high flash temperatures of the braking surface. Less than 0.02% of the lining wear was released as asbestos fibers. The concentration of asbestos fibers in the urban atmosphere, due to brake usage, was conservatively estimated at less than 0.07 X 10-9 g/m3. Based on this upper bound, the use of brakes was judged to be not significant as a source of atmospheric asbestos.

A compact new laboratory friction and wear test machine has been developed. Test procedures have been established for this machine in a constant output (that is, constant friction force) mode of operation. These procedures have been shown to be particularly well suited for quality control of brake lining materials. The test, designated Friction Assessment Screening Test (FAST), has been shown to yield highly reproducible results which correlate well with vehicle performance. The results are highly sensitive to those variations in brake lining properties which are most significant in brake performance.