From the Lojban web site FAQ (http://www.lojban.org/files/brochures/faq.htm):
brivla: any word that can state a relationship among several objects or concepts, and thus be the core word in a Lojban bridi — a "predicate word"
brivla are in the main gismu, lujvo and fu'ivla. A few cmavo can also behave as brivla.
As noted above, these words state a relationship, and not an entire bridi, so a better word is selbrivla, i.e. valsi lo selbri (a word which means a selbri).
Why isn't it brivo?
If vo was a rafsi for valsi for purposes of making a few new gismu (cmavo = cmalu valsi, lujvo = pluja valsi), why not for this one?