Why is it that I can only fill my cooker 1/2 full when cooking split peas?

Dried split peas, lentils and other grains expand as they cook. They also have an outer layer of starch that foams when they are cooked. This foam combined with the expanding grains could clog the valve if you fill the cooker past the half way mark. The foam is also the reason that you use only the "Natural Release Method", that is, remove the cooker from the heat and allow it to release the pressure naturally when the cooking is finished. Using the cold water or quick-release method remove the skins from the beans or grains. When left aside to cool naturally, the beans remain intact, firm on the outside, soft on the inside.