[ngoma]

If there were no leaks in the IP or upstream of the IP, and you pulled thru the return line, likely you could have purged air from the IP within a liter or two. Visualize the capacity inside the IP. You are pulling from the return line outlet, which is at the top of the top cover, so pulling out from the air space there while the fuel is entering via the fuel inlet, displacing the air.

We don't know enough yet to tell where the air is still entering from.
Some possible sources:
IP seals
Fuel filter
Fuel hose connections
Injector spill lines
There is also the possibility that the air is entering at the Mityvac hose connections or its capture cup, especially if you are going all out on the pumping handle and pulling high vacuums.

Does your IP have the transparent inlet hose? If you see bubbles moving along in there while pumping the Mityvac, that tells us there is air entering upstream of the IP.

The next step is to substitute some clear fuel line before & after the IP, and watch for bubbles at both locations while Mityvac-ing.