Vehicle description: trucks.system: service brakes; tractor park valve control knob.consequences of defect: the purpose of the tractor park valve IS to allow thedriver to apply the tractor spring emergency/parking brake independently of thetrailer emergency parking brakes. If the knob IS broken, the tractor parkingbrakes cannot be applied. If parked on an incline and left unattended, vehiclecould roll free and cause accident and injury.