NHTSA Preliminary Evaluation PE86070

SUMMARY OF INFORMATION FROM MANUFACTURER:MANUFACTURER HAS NOT BEENCONTACTED.ANALYSIS:ANALYSIS REVEALS THAT THE PROBLEM IS RELATIVELY UNKNOWN DUE TOILLUSIVE SYMPTOMS.WHEN THE PRESSURE PLATE HANGS UP, THERE IS REDUCEDAVAILABLE STROKE TO THE SLACK ADJUSTER.IF THE HANG UP OCCURS AFTER ABRAKE ADJUSTMENT, BRAKE DRAG, OVERHEATING DRUMS, AND ACCELERATED BRAKESHOE WEAR MAY RESULT.A MAINTENANCE PERSON INSPECTING A BRAKE SYSTEM FORPROPER STROKE COULD EASILY MISINTERPRET A HUNG UP PRESSURE PLATE AS ANIMPROPERLY ADJUSTED SLACK ADJUSTOR.ASSUMING THAT AN ADJUSTMENT WAS MADETO BRING THE STROKE INTO THE NORMAL SPECIFICATION RANGE, THE RESULT WOULDBE REDUCED STROKE, WHICH MEANS REDUCED BRAKING CAPABILITY.SAFETYIMPLICATIONS ALSO RESULT WHEN AN AUTOMATIC SLACK ADJUSTER IS BEINGUTILIZED.MOST LIKELY THE UNIT WOULD SENSE A SHORT STROKE AND NOT ADJUSTUNTIL THE STROKE ACHIEVED ITS DESIGN ADJUST POSITION.THIS TOO WOULDRESULT IN REDUCED BRAKING CAPABILITY.THE PROBLEM REPORTS ARE PROBABLYDILUTED DUE TO THE MISINTERPRETATION OF THE SYMPTOM BY MAINTENANCE ANDSERVICE PERSONNEL UNEDUCATED IN THIS MATTER.PROPOSED ACTION:OPEN A PE.SEND AN IR TO THE MANUFACTURER.CONTACTOEM'S AND FLEETS TO DETERMINE THE SCOPE OF THE PROBLEM.