The shake test doesn't always work. Especially if a carb has been sitting.

To test a brass float, boil a pot of water, remove from stove and let it sit a few minutes so there are no bubbles floating around. While the water is still hot use pliers to completely submerge the float. If there is a hole you'll see bubbles coming from it just like checking a tire for leaks. The hot water is causing the air inside the float to expand, forcing it out of any pinholes in the solder seam. Won't create enough pressure to pop a good float.