I don't own a Taipan Vet, but I wonder if that steel ball goes in the hole and the screw holds it in? The picture you provided looks like there is a seat for the ball to be forced against. You may have to get a bottom tap and clean the stripped threads and maybe replace the grub screw if those threads are damaged. If the threads are damaged enough, you may need to have the fill port body replaced.
Could be. It would be interesting to see that screw. If it's metal-on-metal fit, no seal, it would have to be a darn good fit.
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