  We discuss the dynamics of expanding bubble walls in the presence of massive
dark photons whose mass changes as they cross the wall. For sufficiently thin
walls, we show that there exists a transient kinematic regime characterized by
a constant reflection probability of longitudinal -- but not transverse --
modes. This effect can have important implications for the dynamics of
expanding vacuum bubbles in the early Universe. Most notably, it leads to a new
source of pressure on the expanding interface, featuring a non-monotonic
dependence on the $\gamma$-factor of the bubble walls and reaching a peak at
intermediate $\gamma$-factors that we dub Maximum Dynamic Pressure. When this
pressure is large enough to halt the acceleration of the bubble walls, the
difference in vacuum energy densities goes into making a fraction of the dark
photons relativistic, turning them into dark radiation. If the dark radiation
remains relativistic until late times, an observable contribution to $\Delta
N_\text{eff}$ is possible for phase transitions with strength $\alpha \sim
10^{-2} - 10^{-1}$.
