EMPLOYEE WAS DRIVING THE WATER TRUCK THROUGHOUT THE QUARRY. ON THE THIRD LOAD, HE WAS GOING BACK DOWN INTO THE PIT TO WATER DOWN THE MUCK PILE WHEN STATED THE BRAKES FAILED. HE TRIED TO APPLY THE BRAKES SEVERAL TIMES, EACH TIME THE TRUCK CONTINUED TO GAIN SPEED. IT GAINED SPEED AT A FATER RATE AND HE DECIDED TO JUMP OUT OF THE TRUCK.