In a pressure cooker, brown roast on all sides in oil. Remove roast. Add cooking rack; return roast to pan. Add cooking rack; return roast to pan. Add 4 cups of water. Close cover securely; place pressure regulator on vent pipe. Bring cooker to full pressure over high heat. Reduce heat to medium high; cook for 40 minutes. Remove from the heat; allow pressure to drop on it's own. Remove meat and keep warm. Pour juices into a cup or bowl; skim fat. Remove rack from cooker. Add potatoes, carrots, onion, garlic, salt, pepper, thyme, bay leaves, pan juices and remaining water to cooker. Cover securely; return cooker to full pressure. Reduce heat; cook for 8 minutes. Immediately cool according to manufacturer's directions until pressure is completely reduced. With a slotted spoon, remove vegetables and keep warm. Discard bay leaves. Combine cornstarch and cold water until smooth; stir into the pan juices. Bring to a boil; cook and stir for 2 minutes or until thickened. Serve with the meat and vegetables.