Not too much pressure. Is the needle seated? Trigger going all the way forward? The needle seating in the tip is what stops paint flow so if paint is flowing when pressing the trigger down either the needle isn't seated, or the the tip is split or cracked, or the tip isn't in there or not seated.
Maybe loosen the needle chuck and slide the needle back a little then make sure the air cap is snug and holding the tip in place. When you reseat the needle give it a turn to make sure it's seating in the tip.
Work the trigger back and forth and make sure the follower behind the trigger goes all the way forward with the trigger. If it doesn't maybe put a little lube on the needle or increase the return spring pressure a little.
There is a flat white o ring that goes behind the head but that seals the air flow. When I'm reassembling the front end I tighten the head then set the tip in place, start the air cap and then slide the needle forward but leave the chuck loose while I snug down the air cap. Then slide the needle back a little and reseat it, giving it a slight turn while I do. Then tighten the chuck on the needle.