Crack bones. Put into 7-qt. pressure canner, being careful not to fill above “full” mark. (If using 7- or 8-qt. pressure saucepan, use half this recipe.)
Add soup meat, peppercorns, onions and garlic. Cover with cold water. (Stock will be highly concentrated, so it’s best to add salt later, when you make soup.)
Close pressure canner cover; bring up steam. Exhaust air, close valve when pressure reaches 15 pounds. Adjust heat to hold at 15 pounds. (Good idea to watch for about ½ hour to be sure pressure is maintained steadily.)
Cook under pressure 4 to 8 hours. Remove from heat. Let pressure go down without opening petcock. Cool stock until it can be handled easily, and fat rises to top. Skim off fat.
Pick out bones with kitchen tongs. Use slotted spoon to remove big bone fragments and chunks of fat and gristle; strain through cheesecloth, returning bits of lean meat to stock.