Sounds like it needs new seals I take it its the same as mine manual flush. Follow these instructions.
UK www.thetford-europe.com
Australia www.thetford.com.au
Australia site has stockist list.
FOR UK USERS.
HOME PAGE SELECT ENGLISH {if in Australian forget this selection
NEXT PAGE SELECT CUSTOMER SERVICE (tope line)
NEXT PAGE SELECT USER INFORMATION (left of pic)
NEXT PAGE SELECT REPAIR INSTRUCTIONS (left of pic)
NEXT PAGE SELECT BUILT IN TOILETS [top right of pic]
NEXT PAGE SELECT C2, C3., C4 (or relevant) or select your model
NEXT PAGE PDF Files YOU ARE NOW IN PDF FILE LIST
bob
April 2009