There should be a pound rating written on the side of the cylinder. They start around 10 pounds and go up. I have replaced a few and it will depend on the size or the door and weight as well as the style of door. The Navigator will likely have different doors than the Diplomat. Lots of places online to purchase. Good luck.
As a retired truck driver from BC Canada with 43 years and 0ver 3 million miles I have had this issue with both rusted and frozen brakes. What this person is suffering from is shoes rusted to the drum. You can hit the drum with a hammer, pry with a bar or release brakes and use power. With drums it is easier if you back up but will break loose by going ahead. No one here laying on their side or back under there coach and hitting the drum with a hammer is going to hurt the drum. Recommend when driving in heavy rain and just before parking gently apply brakes while under power until you feel them start to grab just to dry them out.
