that could be 1 of 2 possible faults.
1= The sensor that reacts to air pressure when x amount of water is filled could be faulty, needs replacing or,
2= There is a rubber 'boot' on the bottom of the drum where this sensor lives, could be a split in the rubber or seal, this would cause the sensor to misread the pressure of water and continue filling.
Hope this helps
George from Wolves, September 2008