What is a critical factor in checking the brake drum's friction surface?

The critical factor in checking the brake drum's friction surface is the presence of cracks extending to the open edge of the drum. This is crucial because such cracks can compromise the structural integrity of the drum, potentially leading to failure during braking. Brake drums are subjected to high levels of stress and heat during operation, and any cracks that reach the edge can propagate under these conditions. This can result in significant safety hazards, including brake failure, which could lead to serious accidents.

To ensure the safety and performance of the braking system, visual inspections are important to identify any signs of damage or wear. While factors like rust on the surface, thickness of the drum material, and the color of the drum can provide information about the condition of the drum, they do not pose the immediate danger that cracks do. Rust may indicate corrosion but doesn't necessarily affect the drum's ability to function unless severe. Thickness is vital for performance but doesn't directly imply failure unless it reaches minimum specifications. Color can indicate overheating but is not as directly harmful as a crack. Thus, identifying cracks that could lead to drum failure is the most critical factor in this context.