  In this work, we construct the chiral Lagrangian for a light spin-1 boson $X$
possessing both vectorial and axial couplings to the light Standard Model
quarks $u, d, s$. We then use it in order to describe the tree-level,
model-independent contributions to the $\Delta S = 1$ transition $K^\pm
\rightarrow \pi^\pm X$, which is induced by Standard Model charged currents and
is possibly enhanced by the emission of a longitudinally polarized $X$ boson.
Such a flavour observable is then shown to set the best model-independent
bounds on the diagonal axial couplings of $X$ to light quarks in the mass range
allowed by the decay kinematics, improving the currently available constraints
from beam-dump experiments and collider searches.
