Plug an open syringe into the lever with a bleed hose, half-fill the syringe with water, move the lever so the air can escape, pull the lever a few times, air will come out.
Won't be a leak, no. If you want to see why, half-fill a syringe with water, put your finger over the end of the syringe and pull the plunger upwards. See all that air? That's what's in your brake! It only becomes apparent once you have used the brake, which is why it feels fine immediately after bleeding, but weird after you've ridden it for a bit.