lol
a wise man once told me when looking at any problem
SSS- "start simple stupid" lol
the rotary pump i believe will be a Lucas CAV pump (tractor pump)
the injector lines off the pump will be in a circular pattern off the inj pump head

inline the outlets would be "inline" across the top of the pump and i believe will be a "diesel kiki"

either way if you got air b4 the inj pump its not the problem

look for anything
you might even want to pull the pickup assembly and inspect the pickup tube for rot or a broken weld (water in the tank at some point?)

you may want to see if the lift pump is pumping air in too
see if its weephole looks wet

do you get air before or after the filter?
change the gaskets or base altogether
ive had a base or 2 in the s4s go bad b4

good luck