power off, navigate to the rear, take off rear panel at bottom, by the pump you will find a rubber elbow, unscrew retaining jubilee clip, lower rubber elbow and inside you will find a plastic cup and ball clean this out and re-assemble making sure it fits in lug at bottom of elbow, connect back up and making sure jubilee clip is secure and power up and see if this has sorted your problem out, it did for me (checking and clearing it every 6 months now)
Mr Tibbs, November 2007