Hi J- To the best of my knowledge my best answer would be look to your corners. I can’t imagine dropping your jaw would create leakage. Just trying to do what Greg usually suggests, make it rediculous, place your mouthpiece as you would to play that low c with what you feel the “normal” drop in the jaw would be for you, play it that way, now really drop the jaw and make it ridiculous. When I try this there is such a difference that my corners start to fight the drop and the seal is more pronounced because you really have to use the corners more to not have any leakage. Again like Greg has said, seal your lips tight and try to blow air through a good seal and you’ll turn blue before air will come out. The lips are really strong. So my best suggestion would be MMMahhOOO, your jaw throat and corners are set, place mouthpiece on this and close your eyes, slingshot release holding your corners, directing the air straight out and do it over and over until your brain is fried. That low C is key to everything that follows. Also you mentioned one word in your last post that really sticks out to me. “concentrate”. This is a must whenever you practice or your wasting time. It’s much easier to get into a deep concentration when your eyes are closed and your senses come alive. Just my Humble thoughts for you. Nothing is easy until it is.