  We study second-order hydrodynamic transport in strongly coupled
non-conformal field theories with holographic gravity duals in asymptotically
anti-de Sitter space. We first derive new Kubo formulae for five second-order
transport coefficients in non-conformal fluids in $(3+1)$ dimensions. We then
apply them to holographic RG flows induced by scalar operators of dimension
$\Delta=3$. For general background solutions of the dual bulk geometry, we find
explicit expressions for the five transport coefficients at infinite coupling
and show that a specific combination,
$\tilde{H}=2\eta\tau_\pi-2(\kappa-\kappa^*)-\lambda_2$, always vanishes. We
prove analytically that the Haack-Yarom identity
$H=2\eta\tau_\pi-4\lambda_1-\lambda_2=0$, which is known to be true for
conformal holographic fluids, also holds when taking into account leading
non-conformal corrections. The numerical results we obtain for two specific
families of RG flows suggest that $H$ vanishes regardless of conformal
symmetry. Our work provides further evidence that the Haack-Yarom identity
$H=0$ may be universally satisfied by strongly coupled fluids.
