IllustrationPE87039 · Rear Disc Brake Failure
Opened Jul 14, 1987 · Closed Oct 13, 1987
Status: closed (inferred from source dates) · Service Brakes, Hydraulic
SUMMARY OF INFORMATION FROM MANUFACTURER:VEHICLES INVOLVED ARE EQUIPPEDWITH FOUR WHEEL HYDRAULIC DISC BRAKES.THE ALLEGED DEFECT PERTAINS TO THEREAR BRAKES ONLY AND INVOLVES SEPARATION (DEBONDING) OF THE BRAKE LININGPAD FROM THE STEEL BACKING (SHOE).ALL FIELD REPORTS AND COMPLAINTSINVOLVE FLEETS.THE SPECIFIC NUMBER OF ACTUAL SEPARATIONS IN UNKNOWN.ANALYSIS:DEBONDING RESULTS IN DISINTEGRATION AND POSSIBLY EJECTION OF THEPAD FROM THE CALIPER.IT IS INITIATED BY A COMBINATION OF TIME AND HIGHOPERATING TEMPERATURES, WHICH CAUSE THE ORGANIC RESINS IN THE BONDINGADHESIVE TO BREAK DOWN AND LOOSE THEIR STRENGTH.WHEN THE PAD IS EJECTED,THE CALIPER PISTON WILL MOVE OUT UPON THE NEXT FOOT APPLICATION, FORCINGTHE REMAINING STEEL SHOE TO CONTACT THE ROTOR.FURTHER OPERATION MAYDAMAGE THE PISTON O RING, RESULTING IN BRAKE FLUID LEAKAGE AND PARTIALBRAKE LOSS.SINCE THE VEHICLES ARE EQUIPPED WITH A DUAL MASTER CYLINDERAND A SPLIT BRAKE SYSTEM, A PORTION OF THE BRAKE SYSTEM WOULD REMAINFUNCTIONAL.ALTHOUGH REPORTS ON THIS PROBLEM COMMENCED IN 1983, THERE ARE NO KNOWNACCIDENTS OR INJURIES.PRESENTLY THE MOST SIGNIFICANT AFFECT OF THEPROBLEM IS THE ECONOMIC LOSS TO THE FLEETS DUE TO THE NECESSARY INCREASEDFREQUENCY OF MAINTENANCE.PRODUCTION HAS COMMENCED ON 1988 MODEL YEARVEHICLES USING A NEW PAD WITH IMPROVED MECHANICAL LOCK WHICH ELIMINATESDEBONDING, ACCORDING TO FIELD TESTING.THE NEW PAD SUPERSEDES THE PREVIOUSPAD, WILL BE AVAILABLE THROUGH GM PARTS DISTRIBUTORS IN THE NEAR FUTURE,AND IS EXPECTED TO REDUCE MAINTENANCE FREQUENCY.THIS ISSUE BECOMES SAFETYRELATED ONLY IF IGNORED.PROPOSED ACTION:CLOSE THIS PE.NO SAFETY RELATED DEFECT TREND HAS BEENIDENTIFIED.MAINTAIN CONTACT WITH THE SCHOOLBUS INDUSTRY AND MONITORCOMPLAINT DATA ON THIS ISSUE.===