If you need to show all private brands first, then I would sort on the
private field, with the score being a secondary sort. Make sure
track_scores is enabled (which it might be by default). All your private
brands will appear first, sorted by score, followed by the non-private
brands, also sorted by score.
I prefer using function scores over boosting via boolean queries since the
boost level is consistent. Boosting in a boolean query can turn out not to
be linear due to the way the Lucene scoring model works. With function
scores, you know a boost of 5 will boost the overall score by 5. With the
new painless scripting language, scripts should be cached and therefore the
performance penalty should be negligible.
Cheers,
Ivan