  The form factors for the $K_{\ell 3}$ semileptonic decay are computed to
order $O(p^4)$ in generalized chiral perturbation theory. The main difference
with the standard $O(p^4)$ expressions consists in contributions quadratic in
quark masses, which are described by a single divergence-free low-energy
constant, $A_3$. A new simultaneous analysis is presented for the CKM matrix
element $V_{us}$, the ratio $F_K/F_{\pi}$, $K_{\ell 3}$ decay rates and the
scalar form factor slope $\lambda_0$. This framework easily accommodates the
precise value for $V_{ud}$ deduced from superallowed nuclear $\beta$-decays.
