  We describe expressions for pion and kaon dressed-quark distribution
functions that incorporate contributions from gluons which bind quarks into
these mesons and hence overcome a flaw of the commonly used handbag
approximation. The distributions therewith obtained are purely valence in
character, ensuring that dressed-quarks carry all a meson's momentum at a
characteristic hadronic scale and vanishing as $(1-x)^2$ when Bjorken-$x\to 1$.
Comparing such distributions within the pion and kaon, it is apparent that the
size of SU(3)-flavour symmetry breaking in meson parton distribution functions
is modulated by the flavour dependence of dynamical chiral symmetry breaking.
Corrections to these leading-order formulae may be divided into two classes,
responsible for shifting dressed-quark momentum into glue and sea-quarks.
Working with available empirical information, we build an algebraic framework
that is capable of expressing the principal impact of both classes of
corrections. This enables a realistic comparison with experiment which allows
us to identify and highlight basic features of measurable pion and kaon
valence-quark distributions. We find that whereas roughly two-thirds of the
pion's light-front momentum is carried by valence dressed-quarks at a
characteristic hadronic scale, this fraction rises to 95% in the kaon; evolving
distributions with these features to a scale typical of available Drell-Yan
data produces a kaon-to-pion ratio of u-quark distributions that is in
agreement with the single existing data set; and predict a u-quark distribution
within the pion that agrees with a modern reappraisal of $\pi N$ Drell-Yan
data. Precise new data are essential in order to validate this reappraisal and
because a single modest-quality measurement of the kaon-to-pion ratio cannot be
considered definitive.
