Fiamma cassette toilet cap seal leak

We have a Fiamma Cassette toilet, which (from reading other websites) I think Fiamma stopped making about 20 years ago.  When opening the cassette to put blue in or empty it, the cap has a seal inside.  Recently when I opened it, this seal came out from its normal position inside the cap.  I pushed it back in, and thought everything was ok, but when we drove through some twisty hilly roads, some blue escaped from the cassette (fortunately toilet was unused at this point).  When we got home, I put water into the cassette and sloshed it around, and a small amount came out from under the cap.  I suspect this may mean the seal is starting to perish.  I notice that Fiamma do a spare seal (80mm) for their BioPot toilet.  Does anybody know whether this seal would fit inside the cap of my cassette?  (Having measured the diameter of the opening in the cassette, it does seem to be 8cm)    Or is there any trick to prolonging the life of the seal, or anything I should have done specifically when trying to reseat it?  
Thanks

i assume it is the filler cap seal and not the valve seal that is leaking.try soaking the rubber seal in vegetable oil.rubber seals get brittle with time veg oil helps to revive it.do not use lubricating oil.hope this helps. g.

Yes, it is the seal of the cap that you take off for filling and emptying the cassette.  (Not the one that connects the cassette to the toilet)   Thanks for your suggestion of the vegetable oil.  I will certainly try that, it sounds like it could be a very quick and easy solution.

Don't know where I read it, either on the forum or in the manual, but for some reason unknown to me you should only use olive oil on toilet seals.

sunflower oil is just as good and you can get it in spray cans,very useful for the blade workings and other jobs.g.

Thank you ever so much for the advice.  I soaked it overnight in (standard supermarket-own-brand) vegetable oil, and it worked a treat.  It looked better next morning, and we recently took the van down the same stretch of road where we first had a problem, and there was no leakage at all!

Hi there,

It sounds as if it will need changing in the near future despite the recent recovery.   

This type of screw cap is usually a standard size (water tanks etc) so a wide variety of rubber seals will fit.   

Seek and ye shall find - at an economical price!   

Hope this helps. 

Best regards,

mine's the 1992 original and still going strong.g

Yes, olive oil is recommended by Thetford I think. You can also buy a spray for the other seal, the one which opens when you flush.

Best thing Fiamma did was stop making toilets. I have one which is an absolute nightmare.

Nora