Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
per-brĕvis, e, adj., very short, very brief or concise: orator perornatus et perbrevis, Cic. Brut. 43, 158: perbrevi tempore, id. Verr. 2, 3, 9, § 22.
Separate: altera pars per mihi brevis videtur, id. Clu. 1, 2.
In abl.: perbrevi, adverbially, in a very short time, Cic. Fam. 6, 12, 3.
Adv.: perbrĕ-vĭter, very briefly, very concisely: quid sentiam, perbreviter exponam, Cic. de Or. 2, 58, 235 al.