are you using the correct O-rings? the obsolete o-rings were just flat, "square" o-rings. The revised design o-rings are called "X" o-rings, because they are molded with ridges. If you can imagine cutting a cross section of the ring it looks a little like an "X".
Another thing to check is to make sure the seat is fully tightened, and when you are adjusting the float level, you have to be careful to not press on the float and the needle, because you can deform the conical rubber tip. Also, rarely, one of those plastic floats can absorb fuel, and become too heavy and lose buoyancy.
