Log on to www.thetford-europe.com
click English then customer service and on left list 'Ask thetford'
Or www.thetford.com.au for Australia [yes they paste on forum as well] drop the select English
and look for customers sevice then 'Ask thetford'.
Does sound like an air lock some times removed by bouncing the pump up and down in the water.
Bob
April 2009