  We investigate the isospin symmetry breaking effects within a recently
derived Nambu-Jona-Lasinio related model by fitting the measured pseudoscalar
meson masses and weak decay couplings $f_\pi$, $f_K$. Our model contains the
next to leading order terms in the $1/N_c$ expansion of the effective
multi-quark Lagrangian, including the ones that break the chiral symmetry
explicitly. We show the important phenomenological role of these interactions:
(1) they lead to an accurate fit of the low-lying pseudoscalar nonet
characteristics; (2) they account for a very good agreement of the current
quark masses with the present PDG values; (3) they reduce by $40\%$ the ratio
$\epsilon/\epsilon'$ of the $\pi_0-\eta$ and $\pi_0-\eta'$ mixing angles, as
compared to the case that contemplates explicit breaking only in the leading
order, bringing it in consonance with the quoted values in the literature. The
conventional NJL-type models fail in the joint description of these parameters.
