I've resolved this problem many times by either striking the top of the banjo bolt squarely once assembled and tightened, and then re-tighten it several times. It looks like you won't be able to get a good strike with the caliper bolted in place, so remove the caliper place it on a block of wood or something, and strike the banjo bolt squarely with a good blow. You will know if the strike was sufficient if you can tighten the banjo bolt more.
I've also just tightened, loosen and re-tightened the banjo bolt half a dozen times to get the bolt to seat into the copper washer during the process.
The annealing process eluded to above is another step to get a proper seal. Getting the copper red hot and allowing it to cool without quenching it is a good first step, as well.