Desert Fox Fuel bladders caps leaking

[Bordercollie]

Have 2 newish (1 x 2 years and 1 x 1 year old) Desert Fox fuel bladders. The caps from new did not fit well, seems that the threads on the cap and spout does match 100%.

Both are leaking, got 2 new Desert Fox caps last week and they do not fit at all. Now use them with a checkers plastic bag as a seal under the cap.

Any alternatives, caps, seals?

 

[Leo]

 

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[melior]

 

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[Bordercollie]