In many cases, the bottom of the small tank filler neck is rough and the pressure cap won't seal. It may take a shop to unsolder and put on a new neck to fix this. In addition, Euro caps are a little longer than US caps, which then don't have enough spring tension to seal. Dunno about Aussie caps. The small tank should have the pressure cap while the bigger rear tank get the flat cap. Be sure the small dia hose between the two doesn't have a small air leak or the small tank won't suck water back in during cool- down.
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