If you have broken a needle, you most likely have a burr on your hook somewhere. Burrs cause frustrating top thread breaks. Many times you'll actually hear the "pop" as the top thread breaks. All longarmers should have some of this specialized 800 grit Sand Paper in their longarm tool kit. You can also cut it in half and use it in the hole on the needle plate to sand out any burrs there.

Make sure you give your machine a bath of WD-40 and oil it after you've worked on it with abrasives and be sure NOT to sand the tip of the hook. It must stay sharp and stay at the original angle.