Help with zirk fittings

Question: (I hope this makes sense....here goes)
When pumping grease through the wheel zirk fittings, is it fair to say that as long as grease is flowing back out through the front seals that you haven’t pushed any grease out the rear seals and on to the brake drum?
I’m Under the impression that you keep pumping grease until the old grease starts to flow out the front seals and replaces the new grease