An air valve won't work on a partial vacuum.
At the siphon apex, you could try adding a vertical leg or a secondary loop that extends above the elevation of the active siphon. You will need a valving arrangement, a fill cock on top, or some other arrangment to fill the leg/loop. When filled...
Disadvantages of vertically-mounted split-case over conventional horizantal mounting or vertical in-line pumps:
1. Thrust bearings required
2. Overhead crane not much use for lifting out "top"
casing,impeller-shaft assembly, and other
components. Special forklift or horizontal rig...
