Ask thetford they may tell you what it is.
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Just a thought....like a bike pump if you leave it for a long period the washer that causes the pressure dries out and shrinks so air passes instead of being trapped and pumped out into the tube. This could be the prob I don't know I've yet to come across it with my thetford 4 manual pump.[the old twist type pump. ie twist to flush ]
Bob
July 2008
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