  Masers found in massive star-forming regions can be located precisely in
six-dimensional phase space and therefore serve as a tool for studying Milky
Way dynamics. The non-random orbital phases at which the masers are found and
the sparseness of current samples require modeling. Here we model the
phase-space distribution function of 18 precisely measured Galactic masers,
permitting a mean velocity offset and a general velocity dispersion tensor
relative to their local standards of rest, and accounting for different pieces
of prior information. With priors only on the Sun's distance from the Galactic
Center and on its motion with respect to the local standard of rest, the maser
data provide a weak constraint on the circular velocity at the Sun of V_c = 246
+/- 30 km/s. Including prior information on the proper motion of Sgr A* leads
to V_c = 244 +/- 13 km/s. We do not confirm the value of V_c \approx 254 km/s
found in more restrictive models. This analysis shows that there is no conflict
between recent determinations of V_c from Galactic Center analyses, orbital
fitting of the GD-1 stellar stream, and the kinematics of Galactic masers; a
combined estimate is V_c = 236 +/- 11 km/s. Apart from the dynamical
parameters, we find that masers tend to occur at post-apocenter,
circular-velocity-lagging phases of their orbits.
