I have recently had the same problem and find that the sole plate and the top cover have no gasket. When new the plastic probably formed a sort of gasket,but with the powerful suction this slowly wore out. I made a temporory gasket with tape and this helped. I suggest using something like bluetack, plastercine or playdoe to make a seal due to the need to take apart to clean the brushes etc. It`s worth a try.
maintenance man, March 2009