EMPLOYEE WAS AT A FILL STATION LOADING A 10,000 GALLON WATER TRUCK WITH WATER. THE TANK FILLED AND HE WENT TO THE PUMP CONTROL, PUSHED THE STOPBUTTON AND RETURNED TO THE TRUCK TO REMOVE THE WHEEL CHOCKS. AT THAT TIME HE NOTICED THE WATER FLOW HAD CONTINUED. HE TURNED SUDDENLY TO RETURNTO THE PUMP CONTRO AND HIS KNEE ""GAVE WAY"" AND HE FELL TO THE GROUND.