  We present optical and near-IR spectroscopy (600--1000 nm) of 8 faint
(I$>$18) very red (R--I$>$2.2) objects discovered in a deep CCD survey of the
Pleiades (Zapatero-Osorio et al. 1996). We compare them with reliable cluster
members like PPl 15 and Teide 1, and with several field very late-type dwarfs
(M4--M9.5), which were observed with similar instrumental configurations. Using
pseudocontinuum ratios we classify the new substellar candidates in a spectal
sequence defined with reference to field stars of known spectral types. We also
reclassify PPl 15 and Teide 1 in a self-consistent way. The likelihood of
membership for the new candidates is assesed via the study of their
photospheric features, Ha emission, radial velocity, and consistency of their
spectral types and I magnitudes with known cluster members. Four of the new
substellar candidates are as late or later than PPl~15 (M6.5), but only one,
Calar 3 (M8), clearly meets all our membership criteria. It is indeed an object
very similar to the brown dwarf Teide 1. Calar 3, together with Teide 1, allows
one to compare the spectroscopic characteristics of Pleiades brown dwarfs with
those of old very cool dwarfs. The overall spectral properties are similar, but
there are slight differences in the NaI doublet (818.3 nm, 819.5 nm), VO
molecular band (740 nm), and some spectral ratios, which are probably related
to lower surface gravity in the young Pleiades brown dwarfs than in field
stars. Finally, we propose a way of improving future CCD-based brown dwarf
surveys by using narrow-band near-IR pseudocontinuum filters.
