I think the better work to sight is Richardson's newer one, Royal Ancestry. You can find it in this case easily by looking to John's father, where someone has already given a reference for him.
John appears in Vol. 4 under BREWES 7iii, p.532.
Unfortunately these sources are not wonderful in their citations, giving very little "in line" so that you can see how facts connect to sources, and often relying on non-obvious combinations of secondary sources, visitations etc. This is why many of us double check the discussions (sometimes heated) which occur in public on the SGM newsserver. It is a good thing indeed that Douglas Richardson posts updated ideas and proposals there, and to some extent also sometimes accepts that they need tweaking. In any case he cites:
C. Ch. R. 1 (1903) p.471
Rigg et al. Cal. Plea Rolls of the Exch. of the Jews vol.1 (1905) pp 280, 299; and vol.2 (1971) p.5; and vol.4 (1972) p.9.
Complete Peerage vol.2 (1912) p.307 (under Brewes). This is not a primary source, but you may find that Richardson's work is based on this massive work, which is available online, and it is normally pretty good with giving primary sources.
Cal. IPM vol.3 (1912) pp.240-1. Available on British History Online, although perhaps better to use an exact page from archive.org.
He has a special note concerning the fact that Llewellyn ap Gruffudd was described as a kinsman in 1267-7 which gives a specific source: Edwards, Littere Wallie (1940) p.25.
Paget, Baronage (1957) 90: 1-12 under Braose
Adams, Select cases from Eccl. Courts of Canterbury (1981) pp.138-144
Brault, Rolls of Arms Edward I, vol.2 (1997), p.75