first check these  options 
1 pressure pipe split/detached/hole rubbed in  it 
2 pressure chamber blocked
3 pressure switch faulty 
remove the top of machine find the pressure switch round thing with wires and a clear pipe on it, inspect the small bore clear  pipe for splits or a rubbed through hole.
lastly follow the pipe down to the pressure chamber remove the pipe and inspect and clear the internal hole of all gummed up washing powder refit pipe try a fill. 
if no joy still (Over filling) change the pressure switch
                
 wirecutterman
                
 July 2011