The ornamental design for coaxial pneumatic valve, as shown and described.
FIG. 1 is a top view of our design;
FIG. 2 is a bottom view thereof;
FIG. 3 is a front view thereof, the rear view being identical thereto;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top perspective view thereof; and,
FIG. 7 is a bottom perspective view thereof.
The broken lines shown in the figures are included for the purpose of illustrating portions of the coaxial pneumatic valve that form no part of the claimed design.