Final answer:
The proper hand signal when your vehicle's brake lights don't work and you're stopping is to extend your hand and arm downward.
Step-by-step explanation:
The proper hand signal when your vehicle's brake lights don't work and you're stopping is to extend your hand and arm downward. This hand signal indicates to other drivers that you are slowing down or stopping. It is important to use this hand signal to ensure the safety of yourself and others on the road.