over filling can be caused by,
the pressure switch becomes faulty.
pressure pipe maybe split/detached
pressure chamber hole blocked (this is the likely one)
this chamber is the opposite end to the pressure switch so follow the plastic pipe downwards,from the pressure switch,remove pipe clear the hole refit try for filling what you have done is removed the gunged up accumulated washing powder blocking the hole preventing the air pressure making the switch turn off the water valve
wirecutterman
September 2010