I also used to get air in my injector/injections and tried various methods to stop this (with no real success) until JerryV, one of our long lost forum administrators, told me about pouring plastisol directly into the injector. For me, this was a game changer and I'll never go back to drawing it in.
However, I understand you don't feel safe with this (yet) and may never will, so I will offer some advice that I also received related to your dilemma (but never used because I was happy with the results from pouring into the injector)... try wrapping your plunger piston with some teflon tape (aka plumbers tape) to create a tighter and wider sealing area inside the injector... again, I never used this advice, but it came from another forum member and sure sounds logical. Best of luck!