caravan?

go to thetford website downloads free pdf's then also send thetford the question as to what could be causing tell them you've changed the seal but problem still exists then may come up with a different solution. I woldn't have though it needed to be replaced I've had caravans that old and mine were ok just needing the odd seal and pump.
One though on the cassette is the air control for emptying lets air in the seal may need replacing on that just check out the pdf's first.

Bob
June 2011