I cant imagine what kind of engineer has looked at your machine but he must be a complete pr*tt and so must the bonehead who posted the previous answer. There can only be two answers to this question, either one of the inlet valves needs replacing or it is backfilling from the waste which should not happen as it should have a flap valve on the pump that stops it doing this, it should only cost a maximum of £50 to repair.
powerhousesteve@hotmail.com
April 2006