When cooking expanding foods like grains or legumes, what is the critical maximum fill line?

Half-full.

Proper adherence to internal fill lines is a non-negotiable safety prerequisite when operating a pressure cooker, especially concerning foods that increase in volume during cooking. While the general maximum fill line for many standard recipes is set at two-thirds full, foods that expand, foam, or froth—specifically including grains and legumes—require a significantly stricter limit. For these specific items, the pot must never be filled more than half full. This reduced capacity is vital because it prevents the expanding material from reaching the vent or release valve mechanisms. Blockages in these areas can lead to dangerous pressure build-up or failure to release steam correctly, potentially leading to component failure or hazardous eruptions.