9999 GMC RTS

SUMMARY OF INFORMATION FROM MANUFACTURER:SUBJECT VALVE WAS PRODUCED WITH-OUT AN INTERNAL SPRING TO KEEP THE PLUNGER IN THE APPLY POSITION.ASERVICE BULLETIN WAS PUBLISHED IN FEBRUARY, 1984 CONCERNING THE TIMELYREPLACEMENT OF INTERNAL O RINGS IN THE VALVE WHICH IN TURN WILL PRODUCEADEQUATE FRICTION TO... KEEP THE PLUNGER FROM DROPPING TO THE RELEASEPOSITION.IN MID-1986 A NEW PRESSURE SENSITIVE CONTROL VALVE,INCORPORATING A SPRING TO HOLD THE VALVE IN THE APPLY POSITION WASRELEASED.THIS VALVE WILL AUTOMATICALLY APPLY THE PARKING BRAKE IFPRESSURE IN THE SUPPLY TANK DIPS BELOW THE SET-POINT OF THE VALVE.ANALYSIS:IF THE SUBJECT MAINTENANCE WAS NOT PERFORMED, THE AIR SYSTEMWOULD HAVE TO LEAK DOWN AFTER THE PARKING BRAKE HAD BEEN APPLIED. IF THISOCCURRED AND THE O-RING IN THE VALVE WAS WORN ENOUGH, THE SPOOL MIGHT CREEPFROM THE APPLY TO THE RELEASE POSITION.THE PARKING BRAKE WOULD STILL BEAPPLIED.IF THE ENGINE WAS STARTED UNDER THESE CONDITIONS, THE INDICATORLIGHT ON THE DASH WOULD INDICATE THAT THE PARKING BRAKE WAS STILL APPLIED.IF THE DRIVER THEN DECIDED TO LEAVE THE BUS RUNNING UNATTENDED, THE AIRPRESSURE WOULD GRADUALLY BUILD UP RELEASING THE BRAKE; THE INDICATOR LIGHTSHOULD THEN GO OUT AND WITH THE DRIVER NOT IN THE DRIVER'S SEAT, THIS MIGHTGO UNNOTICED AND THE BUS MAY MOVE IF IT WAS PARKED ON AN INCLINE.PROPOSED ACTION:CLOSE THIS PE.NO SAFETY DEFECT TREND WAS IDENTIFIED.===Read more