The reason to get a pressure cooker is to cook tough meat, and your old rooster fits the description!
Here are the pages of my recipe book relating to pressure fried chicken:
I did the last page sideways to try to get more into the scan, it looks funny but I think you can print it better. There are several pages devoted to chicken, pressure cooking used to be what we did with chicken. Now, there aren't any recipes for chicken in new pressure cookers instruction books. This book is dated copyright 1974, but I do not think it violates copyright to share it here, since the new cookers do not include these instructions and times. I have a small 4-qt and a larger 8 quart cooker. The instructions here apply to a 4-qt Presto with a 15 pound pressure weight. Please be sure to verify the size of cooker you have, the amount of food you put into the cooker must be smaller for the 4-qt size. I prefer the small size when I am only cooking for 2, it pressurizes faster and cools faster. To cook a larger whole old chicken, the large size is better.
Enjoy eating your old Roosters!