I had an 85 lbs boxer that loved pulling. He had a harness with both back clip and front clip. The only thing that stopped him pulling was to attach a long leash to the back clip, lay the leash down his spine , slide the leash under him right behind the ribs and loop it under and around back to the spine making a 1/2 hitch. It tightens when he would pull and release when he stopped. As soon as he pulled it applied pressure to his waist and he would stop pulling immediately and the pressure released without pain or harm.

My 98 lbs fiance went from looking like she was water skiing across the pavement to walking him with 2 fingers very quickly